As you may be aware, upside down stairs and slabs will allow items to pass under them. In the case of stairs, this requires the a certain angle, of course. It's very useful for transport of drops via a water current while limiting the water.

However, it appears to be a bit broken.

I tried making a contraption that kills pigs and then transport their drops under some upside down stairs by water. After a while I started getting a headache as to why they didn't pass under them. Upon further inspection, it appeared that they would sorta bounce off the stairs. As this happens, they also appear to stack (only visually I assume). It also happens if you throw the item.
It works fine with a slabs, but when a pig dies, the items will seemingly completely ignore the slab, and the dropped items will pass right through it. Quite annoying.

This issue was in single player. I don't know if it works in SMP, since I've had contraptions carry items under stairs just fine there. This was in 1.4.2.

Summary:
As of 1.4.5, items will bounce back instead of passing under upside down stairs. Tested in Single Player. Appears to be a collision fault. May also affect slabs.

UPDATE: I've tested this issue in 1.4.2 Single Player. Like mentioned, it worked in 1.4.2 SMP, and this was also the case this time. This confirms that the issue is only present in the most recent version.