As a tickler:
My darkest moment occurred early on in my relationship with my significant other. We were naturally still getting to know one another on that level, so there were still plenty of opportunities for us to get our wires crossed. We were playing around, when I misread her teasing and playful signals for a tickle-fight invitation. It turned out she wasn't in that kind of a mood. To make matters worse, I still hadn't gotten fully familiar with how physically sensitive she is, so the tickling (while gentle) was a lot more intense than I had intended it to be. So, this resulted in a playful moment taking a turn for the worse faster than I could say "What happened?" After apologizing for being a douche-bag, and going over the incident, we figured out what went wrong, and I ended up learning a valuable lesson: When dating a playful, stubborn and extremely ticklish woman; just because she's trying to get under your skin doesn't necessarily mean she's looking to get "punished." Sometimes I just have to let her tease me till I turn purple, and resist the urge to tickle her silly. Not that I'm complaining, I've spent my entire life being teased and bugged by my siblings, so...
As a non-consenting ticklee (I won't ever volunteer to be tickled):
Speaking of siblings. My worst moments as a tickle-victim was at the hands of my baby sister, during a time when she had developed a red-hot tickling streak. For a good while she was on the offensive, and I was her #1 victim. During that period she had managed to discovered my weakest stickle spot, and exploited it relentlessly, honing her skills in the process. It was worse when she got my brothers involved, using them to physically overpower me while she worked me over into silent laughter. So, I basically had to look forward to a solid 10-15 minutes of hardcore (and extremely skilled) submission-tickling every day, without safe-words, from my very own sister. Thank God she grew out of that phase (or at least decided to leave me alone). *shudders*