Not many great terribad experiences for me...
Maybe that time when I was still in my first year in Eve (near the end of it) and was still learning a lot about the game, pvp, FC-ing, alliances, sovereignty, nullsec, diplomacy, and most of all, the social aspect of the game, especially when it came to leading people, organizing things, dealing with human players, etc. With a friend we had a very small corp that we mostly dedicated to intelligence warfare, being part of Aegis Militia, and we just started to develop the new shiney little home constellation we were entrusted by CVA in Providence. AM was at the time a very poor pvp alliance with very few means at its disposal. Even our own novice corp of dedicated/devoted players (5 active max) was probably doing a good amount of money with our invention and trade. We also learned about the Istvaan heist, among other things like that. AM had a bloody cursed history with spies, thieves and sabotages. But it was just a story among many.
Someday we logged in to see that the whole alliance was in a mess and learned that most of the AMC POSes had been sabotaged, among other harmful things. In our own corp we were pretty safe since we all worked in full autonomy, alone. We learned that a player had left the AMC corp as soon as he got access to the POS director roles and other accesses as well. So the whole sov on our constellation was pretty much on the verge of collapse since only our corp in our single system still had functional POSes anchored. AM lost a lot this day, but not in terms of money. What were a few POSes and other materials ? All T2 BPOs and the likes were already gone since the first heists. And as I said the alliance was poor. Not even sure what the spy attempted to do, since we were not at war either. I think it may just have been a way for him to "prove" himself.
The real damage was done to the morale of the group. AM had always been an awesome group of nice people helping selflessly each other. Probably the alliance with the nicest people I have been in (Soratah, Kra Ra, Bastables, the people in OPIM, RaataTech, etc, except Graelyn because he was not here
). They just never completely recovered from the series of heist they had to face. It is understandable, when you work so hard to get all these T2 BPOs and else, and that disappears into smoke in a matter of minutes. So, this betrayal was just another hit on the morale and cohesion. It was only too obvious when a few weeks after the AM executors came to hand it over to us, since they were so washed up that they were unwilling to do anything anymore.
I was not even directly the victim of this since it was not about my corp, but nevertheless I felt bad for the alliance. It is also the thing that made me utterly despise corp heists and thefts. It's the game, and would I have had to deal with that while being older, I would certainly not have taken it so badly. Though what I think of that is mostly disgust. It is not the act in itself, but more the fact that you know the guy for a certain amount of time, spend time with him, the guy is nice and fun to talk to, you share good moments and have a project together, and suddenly you eventually understand that all of this was just a lie, a face among other faces, a fake. I wouldn't like it to happen IRL, and I didn't like it either in a game. Like somehow a game excuses all of this.
(It was not done ICly, between characters. It was done between players)