Egads, I appear to have acquired half-blocks which, when moused over, crash my game in a way unlike any I've ever seen.
They don't even seem to stack with my other half blocks.
This effect appears to persist even after dropping and reacquiring the 45 half blocks; testing with a chest now. We may have the ability to place a cursed chest somewhere if we want to. >_>
This will be the second event of bugged items I've encountered today; Djohaal and I both played with some sugar-cane earlier which wouldn't craft into anything (neither paper nor sugar).
Edit: Blocks placed in a chest in the creeperfree caverns which was then locked with a half block on top and labeled as the cursed chest of creeperfree caverns: experiment with at your own risk. Notes of interaction: mousing over directly crashes the client; to pick up, pick up another item first and replace it; when depositing borked item down again, your client will crash, but the item will still have moved; these effects last through dropping and picking up
Edit 2: fking hell... it's happening to all stone slabs I create. The image changes after creation from smooth slab to stone slab, and at this time the bugging begins. I have a suspicion as to a possible cause, reporting back after further tests.
Edit 3: wood of the same apparent type is also not stacking correctly; I suspect one of the mods is doing something funny with data values or, more likely, damage values used to denote sub-types of materials.
Edit 4: It seems to have been the cobblestone out of which they were made which caused it; at least that's the best solution I've got. The remainder of the cursed cobblestone has been disposed of in magma, as is the dwarven way. Mining some fresh stuff, to do one final test before accepting that as the answer.
5: Still wary, but it seems the cobblestone out of which it was made was the root cause.