My turn?  A friend of mine use to create things with her graphic settings on low for speed. One day she set her graphics on Ultra and to paraphrase her comment, the house she had just created looked like a radioactive ball and could be seen from a mile away.   She also thought her avatar could literally die and she'd have to create a new one.

As for me, well..I use to lose paintings in walls and once I accidentally unlinked a very expensive no-copy sex bed and figured it was now destroyed so I deleted it. I use to unlink my hair a lot too and threw the "destroyed" objects away

As for the dumbest of all dumb things....I use to *buy* paintings and posters not knowing that I could make my own in under 5 minutes for free. Well, 10L for the texture upload but free in Opensim.