Mobs have been seen spawning in secure, fully lit areas and on top of blocks where they should not be able to.

While my back was turned, a zombie appeared in a secure room with no dark areas. Later, I saw a creeper spawn in on top of a right-side-up stone brick staircase, which should not be possible. The game does not seem to be checking the conditions correctly before spawning mobs.

Having creepers spawn on stairs is a huge problem for my server because we use stairs to keep mobs from spawning in some decorated areas that don't look good lit or covered with slabs. A lot of work went into these areas and having mobs spawn there is not fun.