Thanks to you all for writing this!
I'm not very good at making lists, but I've done some extremely stupid things as well:
I've been building backgrounds in my aquaria for years, so a lot of mistakes are made in that period: like getting the 2:1 mix-in amounts for my resin the wrong way and almost burnt my hands! Plain luck it happenend in a glas container that actually tolerated the extreme temperature. It only cracked *after* the stuf was hard enough not to leak everywhere.
Back-up tanks... have MULTIPLE!! (really , I am not kidding)

I absoluely *had* to get my first trio of pumilio before their designated tank was complete* (I broke a door and was waiting for the supplier) but -no sweat: I had a back-up all ready to receive them- which I broke the very day I got the pums home

.
Needless to say they didn't take kindly to the improvised back-up-back-up one...
(that was a couple of years ago and I am still kicking myself).
My leuco's are more forgiving
*judging those now-or-never deals are a pain: I've wanted to keep pumilio since the moment I discovered they were on the planet and there was this nice man who was downseizing his collection and had a confirmed breeding trio of my ultimate favorite ones...
(the only comfort I have is that they already had reproduced)
Converted a chameleontank to house my pums (second time): a lovely piece of work, and for once I didn't screw up the build!
(Overplanned like mad, second guessing myself every step, it took forever!)
It was my first tank that didn't start out as an aquarium, so I wasn't prepared for the relative flimsiness of the glass resulting in vulnerable seams!!
Not sure what happenend (prolly a see-saw move when I dit some work next to it) but -as tradion demands it- everything went perfect up to the point my pums were nestled in, starting to call and THEN I woke to the sound of a slow drip-drip-dripping ...
I still have to fix it/ probably have to tear it apart ->it hurts.
(I really loved the way it turned out and functioned)
The 'back up tank' isn't pretty at all.
=> never put a tank where is isn't protected from forces it isn't built to withstand
And of course: buy a very large tank that you can't get into your house!
I measured everything a dozen times, decided *not* to buy it, but then a friend told me it would fit -> and I believed him!
(Whisfull thinking and to much selfdoubt?? Lethal combination!)
He doesn't even live in this building!!
So I ended up buying it anyway, got stuck (stairs) and had to revert to breaking the viv down to pieces (shattering panes etc.in the process) and am still re-assembling to this day ...