When placing stairs in a formation like so (http://i.imgur.com/LzDAQEC.png),
placing a solid block in front of the outer_right stair makes the top half become invisible from one side, causing the stair to look like a slab (http://i.imgur.com/2wMIdu5.png). The stairs look unchanged on the other side (http://i.imgur.com/4GNOmFn.png), thus creating a block that looks like a slab from one side and stairs on the other.
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Here's a screenshot from another angle. http://i.imgur.com/6sFOlH9.png As you can see, only one of the faces disappears. Not sure that's an intended feature.
I can reproduce it.
screenshots of simplified version
Blah, here's a screenshot from behind that may help you. http://i.imgur.com/2ADZ4fg.png The ones marked with red are normal stairs and the ones marked with blue are upside-down stairs. The other 4 all face towards the center.
Also, I noticed it's direction dependent. Make sure the hole is facing west or north, or it will work fine.
An easy way to reproduce this is to build tiny pyramids out of stair blocks and then surround them by solid blocks like in the screenshots I posted here.
Confirmed.
Confirmed for 14w19a.
Confirmed for 14w20b, see MC-55243
Confirmed for 14w20b.
Confirmed for 14w21b.
Confirmed for 14w25b.
I can confirm this for 14w28b

Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.
This issue has been fixed in 1.8.1-pre3.