Mobs getting spawned over and over. Friendly as well as hostile.

^{Update by Mega_Spud:}

It seems plausible that the mob cap is being ignored.

I tested an area with mob spawners and they continued producing new mobs even though there were many in the immediate vicinity.

According to the wiki:

If, at the time of spawning, 6 or more mobs of the spawner's type are present within a 9×9×9 area, centered on the spawner block, the spawner "poofs" without creating any mobs and then waits for the next cycle.

Screenshot: