diff --git a/validation/src/check.rs b/validation/src/check.rs
index bf37214..8523f6b 100644
--- a/validation/src/check.rs
+++ b/validation/src/check.rs
@@ -47,12 +47,20 @@ impl From<crate::nats_types::CheckSubmissionRequest> for CheckRequest{
 	}
 }
 
-#[derive(Clone,Copy,Debug,Hash,Eq,PartialEq)]
+#[derive(Clone,Copy,Debug,Hash,Eq,PartialEq,Ord,PartialOrd)]
 struct ModeID(u64);
 impl ModeID{
 	const MAIN:Self=Self(0);
 	const BONUS:Self=Self(1);
 }
+impl std::fmt::Display for ModeID{
+	fn fmt(&self,f:&mut std::fmt::Formatter<'_>)->std::fmt::Result{
+		match self{
+			&ModeID::MAIN=>write!(f,"Main"),
+			&ModeID(mode_id)=>write!(f,"Bonus{mode_id}"),
+		}
+	}
+}
 enum Zone{
 	Start,
 	Finish,
@@ -446,6 +454,8 @@ struct MapCheck<'a>{
 	mode_finish_counts:SetDifferenceCheck<SetDifferenceCheckContextAtLeastOne<ModeID,Vec<&'a Instance>>>,
 	// Check for dangling MapAnticheat zones (no associated MapStart)
 	mode_anticheat_counts:SetDifferenceCheck<SetDifferenceCheckContextAllowNone<ModeID,Vec<&'a Instance>>>,
+	// Check that modes are sequential
+	modes_sequential:Result<(),Vec<ModeID>>,
 	// Spawn1 must exist
 	spawn1:Result<Exists,Absent>,
 	// Check for dangling Teleport# (no associated Spawn#)
@@ -514,6 +524,33 @@ impl<'a> ModelInfo<'a>{
 		let mode_anticheat_counts=SetDifferenceCheckContextAllowNone::new(self.counts.mode_anticheat_counts)
 			.check(&self.counts.mode_start_counts);
 
+		// There must not be non-sequential modes.  If Bonus100 exists, Bonuses 1-99 had better also exist.
+		fn count_sequential(modes:&HashMap<ModeID,Vec<&Instance>>)->usize{
+			for mode_id in 0..modes.len(){
+				if !modes.contains_key(&ModeID(mode_id as u64)){
+					return mode_id;
+				}
+			}
+			return modes.len();
+		}
+		let modes_sequential={
+			let sequential=count_sequential(&self.counts.mode_start_counts);
+			if sequential==self.counts.mode_start_counts.len(){
+				Ok(())
+			}else{
+				let mut non_sequential=Vec::with_capacity(self.counts.mode_start_counts.len()-sequential);
+				for (&mode_id,_) in &self.counts.mode_start_counts{
+					let ModeID(mode_id_u64)=mode_id;
+					if !(mode_id_u64<sequential as u64){
+						non_sequential.push(mode_id);
+					}
+				}
+				// sort so it's prettier when it prints out
+				non_sequential.sort();
+				Err(non_sequential)
+			}
+		};
+
 		// There must be exactly one start zone for every mode in the map.
 		let mode_start_counts=DuplicateCheckContext(self.counts.mode_start_counts).check(|c|1<c.len());
 
@@ -550,6 +587,7 @@ impl<'a> ModelInfo<'a>{
 			mode_start_counts,
 			mode_finish_counts,
 			mode_anticheat_counts,
+			modes_sequential,
 			spawn1,
 			teleport_counts,
 			spawn_counts,
@@ -573,6 +611,7 @@ impl MapCheck<'_>{
 				mode_start_counts:DuplicateCheck(Ok(())),
 				mode_finish_counts:SetDifferenceCheck(Ok(())),
 				mode_anticheat_counts:SetDifferenceCheck(Ok(())),
+				modes_sequential:Ok(()),
 				spawn1:Ok(Exists),
 				teleport_counts:SetDifferenceCheck(Ok(())),
 				spawn_counts:DuplicateCheck(Ok(())),
@@ -746,6 +785,15 @@ impl MapCheck<'_>{
 				}
 			}
 		};
+		let sequential_modes=match &self.modes_sequential{
+			Ok(())=>passed!("SequentialModes"),
+			Err(context)=>{
+				let non_sequential=context.len();
+				let plural_non_sequential=if non_sequential==1{"mode"}else{"modes"};
+				let comma_separated=Separated::new(", ",||context.iter());
+				summary_format!("SequentialModes","{non_sequential} {plural_non_sequential} should use a lower ModeID (no gaps): {comma_separated}")
+			}
+		};
 		let spawn1=match &self.spawn1{
 			Ok(Exists)=>passed!("Spawn1"),
 			Err(Absent)=>summary_format!("Spawn1","Model has no Spawn1"),
@@ -824,6 +872,7 @@ impl MapCheck<'_>{
 			extra_finish,
 			missing_finish,
 			dangling_anticheat,
+			sequential_modes,
 			spawn1,
 			dangling_teleport,
 			duplicate_spawns,