diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..0d02b2f
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,84 @@
+use std::{error::Error, fmt::Display, ops::Range};
+
+use crate::world::World;
+
+pub type CompileResult = Result<Vec<CompileMessage>, Vec<CompileMessage>>;
+
+#[derive(Debug)]
+pub struct CompileMessage {
+    pub level: Level,
+    pub message: String,
+    pub location: Location,
+}
+
+#[derive(Debug)]
+pub enum Level {
+    Error,
+    Warning,
+}
+
+#[derive(Debug)]
+pub struct Location {
+    pub file_path: Option<String>,
+    pub board: Option<usize>,
+    pub stat: Option<usize>,
+    pub span: Option<Range<usize>>,
+}
+
+impl Display for CompileMessage {
+    fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+        let level = match self.level {
+            Level::Error => "Error",
+            Level::Warning => "Warning",
+        };
+        write!(f, "{}: {}", level, self.message)
+    }
+}
+
+impl Error for CompileMessage {}
+
+impl CompileMessage {
+    pub fn rich_format(&self, world: &World) -> String {
+        format!(
+            "{}\n{}",
+            self.to_string(),
+            self.location.rich_format(&world)
+        )
+    }
+}
+
+impl Location {
+    pub fn rich_format(&self, world: &World) -> String {
+        let board = self.board.map(|i| &world.boards[i]);
+        let stat = self.stat.and_then(|i| board.map(|b| &b.stats[i]));
+
+        let mut parts: Vec<Option<String>> = vec![];
+        parts.push(self.file_path.clone());
+        parts.push(board.map(|board| board.name.clone()));
+        parts.push(stat.map(|stat| format!("stat({},{})", stat.x, stat.y)));
+        parts.push(self.span.as_ref().and_then(|span| {
+            stat.map(|stat| {
+                let mut line = 1;
+                let mut col = 1;
+                for c in (&stat.code[0..span.start]).chars() {
+                    if c == '\n' {
+                        line += 1;
+                        col = 1;
+                    } else {
+                        col += 1;
+                    }
+                }
+                format!("line {line}, column {col}")
+            })
+        }));
+
+        while parts.len() > 0 && parts[parts.len() - 1].is_none() {
+            parts.pop();
+        }
+        let parts: Vec<_> = parts
+            .into_iter()
+            .map(|x| x.unwrap_or("???".into()))
+            .collect();
+        parts.join(", ")
+    }
+}
diff --git a/src/labels/parse.rs b/src/labels/parse.rs
index 7d36a55..6650922 100644
--- a/src/labels/parse.rs
+++ b/src/labels/parse.rs
@@ -3,7 +3,11 @@ use std::ops::Range;
 use compact_str::CompactString;
 use grammar::Tag;
 
-use crate::{peg::ParseState, world::Stat};
+use crate::{
+    error::{CompileMessage, Level, Location},
+    peg::ParseState,
+    world::Stat,
+};
 
 pub type ParsedStat = Vec<Chunk>;
 
@@ -24,7 +28,7 @@ pub struct LabelName {
     pub local: Option<CompactString>,
 }
 
-pub fn parse_stat_labels(stat: &Stat) -> ParsedStat {
+pub fn parse_stat_labels(stat: &Stat) -> (ParsedStat, Vec<CompileMessage>) {
     let code = &stat.code;
     let mut parser = ParseState::new(code);
     assert!(
@@ -33,6 +37,25 @@ pub fn parse_stat_labels(stat: &Stat) -> ParsedStat {
         code
     );
 
+    let mut messages = vec![];
+    for cap in parser.walk_captures() {
+        match cap.kind() {
+            Tag::WarnTrailing => {
+                messages.push(CompileMessage {
+                    level: Level::Warning,
+                    message: "Trailing characters at end of line".into(),
+                    location: Location {
+                        file_path: None,
+                        board: None,
+                        stat: None,
+                        span: None,
+                    },
+                });
+            }
+            _ => {}
+        }
+    }
+
     // Find all #Label captures and record which ones were references
     let mut label_captures = vec![];
     for cap in parser.captures() {
@@ -83,7 +106,7 @@ pub fn parse_stat_labels(stat: &Stat) -> ParsedStat {
     if last_index < code.len() {
         result.push(Chunk::Verbatim(code[last_index..code.len()].into()));
     }
-    result
+    (result, messages)
 }
 
 mod grammar {
@@ -139,7 +162,7 @@ mod grammar {
                 "zap" sp message
             ) /
             message // shorthand send
-        ) s eol; // note: this allows trailing (ignored) whitespace
+        ) warn_trailing eol;
 
         //
         //  Common definitions
@@ -205,6 +228,9 @@ mod grammar {
             &'t'..'w' ("tiger" / "torch" / "transporter" / "water")
         ) eow;
 
+        // Warnings
+        warn_trailing = (#WarnTrailing:(!eol ANY)+)?; // TODO: Document precedence rules
+
         //
         // Generic helpers
         //
diff --git a/src/labels/process.rs b/src/labels/process.rs
index fe870a2..7957050 100644
--- a/src/labels/process.rs
+++ b/src/labels/process.rs
@@ -1,19 +1,31 @@
 use compact_str::CompactString;
 use rustc_hash::FxHashMap;
 
-use crate::world::Board;
+use crate::{
+    error::{CompileMessage, CompileResult},
+    world::Board,
+};
 
 use super::{
     parse::{Chunk, ParsedStat, parse_stat_labels},
     sanitize::Registry,
 };
 
-pub fn process_labels(board: &mut Board) {
+pub fn process_labels(board: &mut Board) -> CompileResult {
     // Parse stats into chunks
+    let mut messages: Vec<CompileMessage> = vec![];
     let mut stats: Vec<ParsedStat> = board
         .stats
         .iter()
-        .map(|stat| parse_stat_labels(&stat))
+        .enumerate()
+        .map(|(index, stat)| {
+            let (parsed_stat, mut new_messages) = parse_stat_labels(&stat);
+            new_messages
+                .iter_mut()
+                .for_each(|m| m.location.stat = Some(index));
+            messages.extend(new_messages);
+            parsed_stat
+        })
         .collect();
     let mut registry = Registry::new();
 
@@ -39,6 +51,8 @@ pub fn process_labels(board: &mut Board) {
             .collect();
         old_stat.code = new_code;
     }
+
+    Ok(messages)
 }
 
 /// Resolve ".local" labels to "name.local" form.
@@ -244,28 +258,28 @@ mod test {
     #[test]
     fn test_label_sanitization() {
         let mut board = board_from_text("tests/labels/sanitize.txt");
-        process_labels(&mut board);
+        let _ = process_labels(&mut board);
         assert_snapshot!(board_to_text(board));
     }
 
     #[test]
     fn test_anonymous_labels() {
         let mut board = board_from_text("tests/labels/anonymous.txt");
-        process_labels(&mut board);
+        let _ = process_labels(&mut board);
         assert_snapshot!(board_to_text(board));
     }
 
     #[test]
     fn test_local_labels() {
         let mut board = board_from_text("tests/labels/local.txt");
-        process_labels(&mut board);
+        let _ = process_labels(&mut board);
         assert_snapshot!(board_to_text(board));
     }
 
     #[test]
     fn test_namespaces() {
         let mut board = board_from_text("tests/labels/namespaces.txt");
-        process_labels(&mut board);
+        let _ = process_labels(&mut board);
         assert_snapshot!(board_to_text(board));
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 2c76a81..8fbf229 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,4 +1,5 @@
 mod encoding;
+mod error;
 mod labels;
 mod peg;
 mod preprocess;
@@ -36,8 +37,22 @@ fn main() -> Result<(), Box<dyn Error>> {
     }
 
     // Resolve labels to proper ZZT-OOP
-    for mut board in &mut world.boards {
-        process_labels(&mut board);
+    let mut messages = vec![];
+    for (i, mut board) in world.boards.iter_mut().enumerate() {
+        let mut result = process_labels(&mut board).unwrap();
+        for message in result.iter_mut() {
+            message.location.board = Some(i);
+            message.location.file_path = Some(world_filename.clone());
+        }
+        messages.extend(result);
+    }
+
+    // Print diagnostics
+    for (i, message) in messages.iter().enumerate() {
+        if i > 0 {
+            println!();
+        }
+        println!("{}", message.rich_format(&world));
     }
 
     // Try to write a modified world file
diff --git a/src/preprocess/peg.rs b/src/preprocess/peg.rs
new file mode 100644
index 0000000..e69de29