My weird one is from when our daughter was tiny we went to visit and we forgot the collar of her avent bottle (you know the white thing that screws the teat thing up to the rest of the bottle and keeps it all in place). I said I'd go into the village and buy a bottle but mother in law wouldn't have any of it and produced an Avent collar from a bottle she had, presumably from when her other grandchildren were younger. I said "are you sure, thanks, I'll get it back to you in the post". So we went home a few days later still with the borrowed "collar" which I would post back (they live several hours away so we borrowed it to have a bottle for the car)
We arrived home and DH rang her to say we'd arrived safely (they like about a 5 hour drive away) and she said "have you posted the collar yet?" and DH said, uh, not yet, we need to find a padded envelope and go to the post office (it was about 11pm at night on a Sunday at this point and we'd JUST WALKED THROUGH THE DOOR!) and she said "ok".
The next day I was in the supermarket and bought a small padded envelope and put the collar in it ready to post, but I needed to go to the post office and figured I'd do it the following morning (I had 2 youngish babies at the time and ran out of time that day).
So the next morning I duly went to the post office and posted the collar back to MIL. It probably arrived if not the next day or the day after that but that night, we had a panicked MIL on the phone asking where her Avent collar was. DH reassured her that it had been posted that day and that it would arrive shortly.
So whilst there is nothing wrong really about wanting something back that you have lent someone, the sheer amount of panic in her voice about this round piece of white plastic was phenomenal! Whilst I had every intention of returning it, bear in mind she had no use for it - her family was complete, there were no more grandchildren to be had and no other babies in the family, its kind of strange that she even had a bottle in the first place, though she probably hoards stuff, fine. Me and DH were completely baffled about the whole thing and regretted even borrowing it but it would have been seriously weird to refuse to either even with the benefit of hindsight.
Then DH said that when she got the bottle collar, she'd produced it from a bottle in her bedroom, not from the kitchen. So where she keeps her stuff is her call but all the other "kitchen" type stuff is either in the kitchen or dining room. So DH and I had a fine old time giggling about the possibilities of why this Avent bottle collar was so important to her, and why she kept it in the bedroom. He mused that she might be one of those crazy ladies with the reborn dolls, but I prefer the theory that she feeds FIL his nightly Horlicks with it 