Epic Tank Errors

Well, it's been quite a morning. Coming to the conclusion that I am nothing short of a total muppet has been quite enlightening. I am far from happy with what I have done, but decided to share the story in the hope that others could learn from my mistakes, and in the hope that others would also document some of their errors (if any) to make me feel a bit better.
A few weeks ago I replanted my tropical 125L tank with real plants. The aquabase went in, followed by a substantial amount of substrate. Everything went well and the glass bloodfin tetras settled back in, then I got some silver hatchets and a little bristlenose plec. All was good. I spent a bit of time on the CC to look at stock levels, then had a good look at the tank. I had gone a tad overboard with the substrate and realised that if I had not used quite so much I would have more space available. Last night I moved the fish, heater and filter to the temporary tank and moved the majority of the plants to the plant tank.
The plan for this morning was to remove the water and some of the substrate, then start again.This is where it went very wrong.
As I was removing the water I thought I saw some movement. I thought perhaps it was bits of dead plant, but on closer inspection it turned out to be a lone glass bloodfin who had been in the tank all night without a filter or heater.  I had thought I was capable of counting to eight, but apparently not. It has been moved to the temporary tank with the others, but I am very concerned that my actions have caused it a lot of distress and that it's chances of survival are not what they should be. 
I have continued with my tank plan, removed most of the water and an entire bucket of substrate, and have started to refill the tank. I'm going to replant, but leave the fish in the temporary tank for a few days because they've been through enough for now.
When I do move the fish back into the 125L tank, if there are any fatalities I will know that I have made a huge mistake this week with my fish. I'll be keeping my fingers crossed for the health of my fish, and have learned a big lesson about planning tanks, but could still kick myself for my stupidity.

We all make mistakes, Donna. You no doubt wouldn't have had clear water in the disturbance of catching the others so it would have been hard to spot, especially if it's hiding from capture... Fingers crossed the wee fella survives. Do you know how low the temperature had got to? X

I've forgotten to put the heater back on and there were no serious repercussions overnight, this time of year the ambient temp indoors is probably around 22C so don't worry too much Donna, there was probably enough bacteria on the hardscaping and tank walls etc to deal with whatever ammonia 1 lone fish produced.

I concur absolutely with the above. Fingers crossed all will be fine.

I've done far worse (no time to relay examples at the mo as I only have 5mins or so to be on here, and also don't want to upset myself thinking about them in too much detail). In fishkeeping, it's so easy to make an oversight despite normally being extremely diligent or to do something with the best of intentions only for hindsight to make it seem completely the wrong action, then spend weeks/months beating oneself up about it which doesn't do any good at all.

I went on holiday once to the South of Devon for a week... In that time my fish back home suffered a power cut which lasted 3 days. Unbeknownst to me when i returned.

When i returned, i checked everyone and their mothers to make sure they were ok, the fish that is! Which thankfully they were.

The fish looked to be in good health and a day later my guppies gave birth We had 13 baby guppies.

Freak incident turned out not to be so detrimental after all

Thanks all for the support, information and stories.
I don't know how low the temperature dropped as I'd also moved the thermometer, but the little fella was a bugger to catch, so I'm hoping that he was still feeling quite active. I was faffing with the tank until quite late last night, so the filter and heater were moved at around 11-11.30. I found the tetra before 10 this morning, so I hope that the time was short enough not to have done him too much harm.
It was the tank with the baby bristlenose. He had been moved to the temporary tank the night before and was the easiest one to catch as he has hidden himself on the underside of a leaf, so I put a plastic bag over the plant and moved the whole thing.
I'm feeling a bit better about things now, and I guess time will tell whether or not the fish feel the same way.

The apisto incident sounds quite unpleasant. I've not had to consider clove oil yet, but I'm guessing it's only a matter of time when keeping fish for a long time. Not only do accidents happen, but fish get very ill, or very old, and sometimes options are limited.
I try not to think about it.

I went for a coffee with the boss this afternoon, so had a mosey to the fish place near work that I don't normally go to because it's smaller and more expensive than the other places. The most sensible thing I've done for a while is leave the shop today without buying some figure eight puffers, who swam to the front of the tank and looked adorable with their big eyes and fluttery fins. Darn you puffers, with your goofy faces and puppy dog eyes.