Maybe we need a thread for dumb goofs! Here's just one of mine.
Many years ago I was new to souping my own negs, and noticed that hypo smelled like pickle juice. So I didn't label the developer or the hypo and used my nose to identify the hypo. Identical storage bottles. You can see where this is going! So I grabbed a bottle and measured out my developer and poured it into the daylight tank, banged it, and started to agitate... and notice a faint pickle smell through my head cold. Yep! Good, fresh hypo, too. Cleared that film in seconds!
Ever since I've been careful to label everything. I haven't made that error since, but there are always lots and lots of others.