Spawner mobs spawning more then they should when their chunk is unloaded and loaded
If a spawner spawns a mob and that spawner and mob are then unloaded then the spawner no longer recognizes the mob as part of the spawner and spawns another mob in even though the mob cap has already been reached.
The "primitive" fix to this issue that I've found is force-loading the chunk the spawner is in, but of course, this takes up more resources from the server.