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.
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I've tested it, and the issue was not present in previous versions. Updated.
CubeTheThird
Please attach a screenshot of your setup.
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Screenshot 1 shows a simple setup that uses water to attempt pushing an item under some stairs. In this case, one wall is exposed. This is important due to what happens in Screenshot 2.
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After bouncing around for a while in Screenshot 1, the item is eventually pushed to the side in Screenshot 2.
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In Screenshot 3, the item is enclosed. In this setup, the item will simply bounce around indefinitely, often several blocks away if not for the water current. Rarely, the item will finally pass under the stairs, but this can take several seconds.
kumasasa
Apparently the items bounce on stairs regardless if regular or upside down --> duplicate of MC-1720
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Finally, we have Screenshot 4. This is a test to show that it isn't exclusive to water.
Item 1 was thrown directly at the middle opening of the stairs in an attempt to get it through. The result was that it was pushed to the side, towards Item 2, which was thrown at a normal block.
If Item 2 is thrown first, Item 1 will land exactly on top, essentially stacking the two. You cannot tell there is more than one item, as the animation will also sync perfectly.
Note that if Item 1 hits the top of the stairs, it will act as if hitting a regular block like Item 2 does.
Item 3 was thrown like Item 1 but at a slab (aiming for the opening), and passed under without problems.