I'm not talking about experimenting or learning things the hard way, I'm talking straight up "I shouldn't have done that..." - pouring bleach in your colored wash, seeing how far you can walk with your eyes closed, pushing the red button that says "Do not push"...
I think mine was tonight. I had a Dorymyrmex insanus escapee the other day, still haven't figured out how (lid was probably not on tight). During the day I start thinking, what if there's some other gap and more get out? I come home and crap! They're all over the table! I start trapping each and every one, wondering how they got out. After a few minutes of inspection, I realize those are just the annual Linepithema humile invasion. Forget those turds and break out the poison. After feeding time, I see two that are not L. humile wandering about. Uh oh. I catch them and dump them in the Dorymyrmex nest. Mission accomplished.
About 10 minutes later, while watching the Crematogasters, I think, wow, they sure look like the escapees. Oh crap... Yes, I dumped two stray Crematogasters into the wrong nest. Thankfully they seemed to know they were out of their element and weren't hard to relocate. And the Dorymyrmex were too busy moving brood from under the feeding dish I just removed to realize they had invaders. It helps to #1 keep lids on tight and #2 identify before dumping!
If you claim to not have made a mistake, you're either lying or not immersed in the hobby