I sent in a bug report (two actually) of when this bug happened to me. Bug #379054 describes time and location in detail:
Kuula, NCI Plaza, autoreturn set to 120 minutes, August 15, 2006
I teleported there at around 03:47 am SLT and rezzed a big plywood box as a wall (created new, not taken from inventory). Then I rezzed a number of scripted panels and a target pointer for a particle stream and started playing with the scripts.
At 03:59 am SLT, all the items, except for the plywood box, were returned to me. It's interesting, that exactly this object stayed in place.
Trying to reproduce the bug, I retraced my steps, ported out of Kuula and back in again at around 04:16 SLT, set up the big box again and rezzed the panels. This time it took much longer, but at 04:47 am all got returned to me - still except for the plywood box.
So either the bug only happens for scripted objects, or it only affects inventory objects, not newly-created ones.
I will try to reproduce it again, this time with several new objects, renamed ones and rezzed ones. We'll see, what happens next.
[UPDATE]
It seems the bug only happens with inventory items rezzed on the parcel. Newly created ones won't be affected. It is not important when you rez them, it happens at certain intervals, as my objects and those of another user were autoreturned at the same time.