I have not confirmed this bug for every version, but it's very likely that it's been in MC since top slabs were added.

I expected the waterdrops/lavadrops to drop from the end of the block.

Instead, they drop a half block down as shown in the picture.

To Reproduce:
1. Open a world
2. Make a "cage" with a top slab as floor
3. Put water or lava in it