Mobs spawn in too light spots from spawners as the spot is just cleared.

Mobs spawning from spawners will have a chance to spawn on a block with it's light level above 7 just the moment the place is cleared from any other blocks.

 

Reproduce:

Find a natural dungeon (preferably a zombie spawner, or recreate such spawner room under ground in a dark spot with enough room)

Now place torches to light up the dungeon room until no mobs spawn when ```{Delay:0}``` is reached.

Now place 2 blocks down ontop of each other near a torch and (mine them up in survival) break them, several times, and a mob will spawn as soon as the block is mined.

See the video linked below for proof/detail.