PVF: Document that preparation cannot lead to disputes (#6873)

* PVF: Document that preparation cannot lead to disputes

* Add warning for deterministic errors

* Fix warning

polkadot/node/core/candidate-validation/src/lib.rs

@@ -623,7 +623,7 @@ where
 		.await;
 
 	if let Err(ref error) = result {
-		gum::info!(target: LOG_TARGET, ?para_id, ?error, "Failed to validate candidate",);
+		gum::info!(target: LOG_TARGET, ?para_id, ?error, "Failed to validate candidate");
 	}
 
 	match result {
@@ -638,9 +638,16 @@ where
 			))),
 		Err(ValidationError::InvalidCandidate(WasmInvalidCandidate::PrepareError(e))) => {
 			// In principle if preparation of the `WASM` fails, the current candidate can not be the
-			// reason for that. So we can't say whether it is invalid or not in addition with
+			// reason for that. So we can't say whether it is invalid or not. In addition, with
 			// pre-checking enabled only valid runtimes should ever get enacted, so we can be
-			// reasonably sure that this is some local problem on the current node.
+			// reasonably sure that this is some local problem on the current node. However, as this
+			// particular error *seems* to indicate a deterministic error, we raise a warning.
+			gum::warn!(
+				target: LOG_TARGET,
+				?para_id,
+				?e,
+				"Deterministic error occurred during preparation (should have been ruled out by pre-checking phase)",
+			);
 			Err(ValidationFailed(e))
 		},
 		Ok(res) =>