In our software, a minority codepath sometimes reported database deadlocks. Nothing critical but it littered the ops error logs and probably displayed error messages to a few customers. So I added a pessimistic exclusive lock to a query which basically solved the deadlock problem (not a great solution but it worked). However, what I missed was the query, even though in a minority code path, was touching another table used basically in all hot-path queries. So basically the code seemed to work fine until deployed to all servers when all operations of the whole cluster got basically serialized through this single lock. So, yeah, database locks can bite you hard!