Item frame items disappear when hitting items. Item pops out but disappear.
Related issues
Comments
migrated
Sonicwave
Probably what Rickard described above, which is MC-3706.
Yes, I don't think they really disappear but they glitch through blocks and have a really weird behavior causes them to end up in places they shouldn't really be.
I've had items flying through the roof and gliding down on the ground below. They can be very hard to find sometimes.