IMPORTANT - I was only able to observe this bug in a server, as the lag is probably somehow related.

When placing a slab on the side of another upper slab, sometimes the slab will be placed on the lower half of the block, not the upper.

This bug was one of the first, being MC - 8, but was then declared resolved.
https://bugs.mojang.com/browse/MC-8

The easiest way to replicate this is by placing a top slab, then place the cursor as low on the side of the slab as possible. Finally, place a new slab and quickly move the cursor downwards.

This has been a bug since pretty much the beginnings of the game, and should really be fixed as it can bother many players, such as I, that are attempting to build a creation using slabs, especially in survival mode.