Bad ideas, gone wrongs & mishaps

The liquid sensor thread got me thinking.

A lot of you guys have built some very impressive, clever and complex systems. You all post them, we admire them, and the show goes on.

For all of you that have a couple dozen batches done on your systems, would you please take a moment to consider what, in retrospect, you would change? I think that would be at least as valuable the build threads themselves.

Did you design something wrong, or unsafely? Did something blow up? Or something you thought would be awesome turn out to be disappointing? Do you wish you had implemented a XXXXXXX?

For my system I built a HERMS system with a control panel and PID/Temp sensor. I am getting the hang of it now but I thought it make brewdays easier. For the most part it has been more of a pain in the ass so far. I guess it takes a while to figure things out but some days I wish I just kept it simple! Also for the conrtol panel my wiring job was less than professional and i've poped my GFI a few times. Scary stuff, I got it to work good now but took some tinkering. This goes to show more complicated isn't always the best.