In KKND Extreme, building my base structures and defences so tight that when I finally decided to attack the enemy base, I couldn't move my forces out of my home base, as the friendly structures were blocking the exits.

Sure you could destroy the friendly structures that were blocking you, but in many levels you were so tight on resources that you didn't want to have to rebuild anything. Hence, restarting the whole level might be in order.

F8 for load is better, that way saving is on the right of a gap and loading on the left of a gap... prevents mistakes.

I use F5-F9 on purpose to avoid pressing the wrong key, because the range is even greater than with F8-F9, and yet sometimes I mess anyway!

I've been replaying Starfleet Command III this last couple of days...

6 or 7 missions into the 3rd campaign, Federation, you're assaulting a Romulan sector, with a Battlestation, 4 defence platforms, a Romulan BC and a few other smaller ships...

It's a BITCH.

Anyway, you're supposed to kill the base, and drive off the BC - her commander is an important Quest person object, so no killy-killy....she's supposed to run away when you knock her hull to 50%.

4 or 5 attempts in, and manage to capture two DPs, kill the base, but for some reason, the BC doesn't run away when damaged, and it's just sat, no movement, in range of one of the DP's guns...which, with what's left of my fleet, pounds on it. It gets to all weapons, power and systems dead, and minimal hull, and for TWO FRIGGING HOURS straight, just will not die, no matter how many overloaded weapons and torpedoes we throw into her...

DP can't be told to not attack it, so it can't repair much before it's shot at again....laying anti-matter mines over it does diddly, too.

ARSE!!

Now got to redo the frigging thing over. Again! And try not to get my arse kicked....

Heh, I just thought of a rather idiotic thing I did in Perfect Dark for the n64. So, I was in the first level of the game and sneaking around being sneaky. I see a scientist facing away from me, so I switch to my fists to stealthily take him out. However, I forgot that a little while ago I had picked up a rocket launcher so my weapon order was slightly different. So, instead of switching to my fists I accidentally switched to the rocket launcher and, before I realized what was going on, fired it point blank into the scientist's back and blew us both to pieces.

In most games with quicksave either quicksaving at death or quickloading instead

One that I repeat over and over, never learning the lesson: not spending the found gold. I keep thinking that the stuff in the store is only an incremental improvement (of course it is, that's the point of loot progression) and that I'll simply find in the next dungeon the +3 Belt of Ogre Flatulence to replace my +2 belt, so there's no need to waste gold to buy the better version. Meanwhile, I struggle through until I do find that item. Repeat for the next loot progression.

When I end the game I retire home to my humble village, only to face the Occupy Faerun protesters decrying my hoard of unspent gold.

Let me add my own variation on the theme (shooters): save the ammunition of the best weapons for the foreseeable future bad times, using only the starting pea-shooter and company (and counting every bullet)...

... and then, halfway through the game, the plot requires that my character is captured, falls into a lake, etc. and have to start again the arms race without having ever used the wonderful weapons and ammunition excess that had collected before the incident.

I used to do that, too, but I figured it was pretty stupid to not take advantage of more fun weapons. Now I use them in a "what's most effective in this situation/against these enemies?" mindset. It's more fun to vary them, and even if you ran out of ammo, you'll find it later.

Yes, I do that now (but learn it cost me a few painful experiences). And I have to keep watching me for me not to repeat it as soon as I get distracted.

I do that all the time. I always save any limited resource that I perceive as valuable (be it ammunition, money, healing potions or whatever), only using it whenever I feel that I absolutely have to (and even then I won't use them until after a few reloads). So in almost any CRPG that I play, I end up with so much money by the end that I could buy a kingdom or two. The one exception is for permanent upgrades, even permanent upgrades that don't make much sense. If it is permanent, I'll probably get it (what do you mean my fighter won't find an intelligence boosting elixir useful? It raises his int permanently, does it not? )
And for healing potions, I'll often end up with a lot of them, and won't start to use them until I start getting better healing potions (or similar), so basically always being a few levels behind in my potion use. Things that boosts my character(s) like potions that boosts armor will almost never get used, because "you never know when these might be useful".

LOL - and "I might not find more of these so I better save them for that really tough battle." Meanwhile, because I'm not using the potions, they're all really tough battles.

Playing Mount and Blade Warband Prophesy of Pendor mod and going rouge and attacking a D'Shar city to take it over...after doing so 1000+ army from my old allies comes knocking on my door..... it was an epic battle indeed...

I guess mine involves Fallout 3, although technically it wasn't an "in-game" mistake. A critical mistake none the less though.

I played hundreds (not gonna admit to how many, it's embarrasing) of hours, finished all but one of DLC quests and the final game quest, made it to level 30 (max)..... and sent my PC back in for repair.... and forgot to back up my save game file.....

Yes, I'm still crying.

That also happened to me but with Dragon Quest VIII. I'd literally invested +100 hours on the game, got to the final dungeon, only for my PS2 to break. Never got to fight the end boss or see the ending because of that.