convert to warn as this is possible when there is a slashing event

substrate/frame/staking/src/pallet/impls.rs

@@ -1903,12 +1903,12 @@ impl<T: Config> Pallet<T> {
 					})
 					.collect::<Result<Vec<_>, _>>()?;
 
-				// we take total instead of active as the nominator might have requested to unbond
+				// We take total instead of active as the nominator might have requested to unbond
 				// some of their stake that is still exposed in the current era.
-				ensure!(
-					sum <= Self::ledger(StakingAccount::Stash(nominator.clone()))?.total,
-					"nominator stake exceeds what is bonded."
-				);
+				// This can happen when there is a slash in the current era so we only warn.
+				if sum <= Self::ledger(Stash(nominator.clone()))?.total {
+					log!(warn, "nominator stake exceeds what is bonded.");
+				}
 
 				Ok(())
 			})