Oh Lord, where would I begin?
My "Oh NO!"s typically are the product of my battery dying before saving. This occurs because I have a SEVERE case of Red/Green colorblindness. It is very hard for me to tell when my power light changes from Green to Red. I always rely on my sons to tell me that my light is red. That does not come in handy, however, when I am out on the road. It is typically a hit or miss for me.
I remember doing the soft-reset in FRLG to finally get Mewtwo with a Modest Nature, only to have my battery die and all that hard work lost. I remember in the Battle Frontier, battling the last trainer before facing Anabel for the Gold Symbol in the Battle Tower and my screen went black. I could give numerous more examples, but those are the most aggravating.
I also mixed records between my Ruby and Emerald after trading my 6 Clefairy to Ruby to set up a Secret Base full of them on my Emerald version. The problem is that I restarted a new game on my Ruby before trading the 6 Clefairy back to my Emerald version. So I had to work to get 6 new Clefairy on Emerald.
Since I do a lot of breeding, I will relase a ton of babies. There have been a few instances where I have released the wrong one and didn't realize it until after I have saved.
I have also gone through the painful ritual of setting up a Secret Base on Ruby, mix records with Emerald, start over a new game in Ruby, go to the Secret Base in Emerald and find out that I had the wrong Pokes on the Ruby team, making that base absolutely useless.
I could go on and on, but this post is depressing me, quite frankly.