In the run the group had to break into a Proteus-AG and insert an operative. The run went well, very well I couldn't believe how well everything went.
Second run had them retrieving the operative. If you are familiar with the scenario you know it can be extremely easy. Unfortunately for the group they went in the hard way. And as good as the first half went, this was far in the opposite direction. They got down in the facility and never made it across to the Arcology. Tripping every damned alarm they found in the landside facility. This resulted in a very nasty fight to gtfo and it was hell. Right as they were about to make it out, the fast shooter of the group. (Normally a very careful player) Chunked his grenade up the stairs following it as fast as he could go.
now on the GM side, it had been established through the security cams (yeah that was also one of the alarms tripped... and not taken care of) that the group was very well supported in the Magic arena. Specifically using alot of Improved invis. knowing this the 6 sec guards were waiting at the head of the stairs fully prepared with ultra vision and in their light sec armor. The grenade went off, only distracting 2 security (out of 6). None were injured and I rolled for surprise and perception. The four guards saw and opened fire. (2) 3rnd bursts each. Needless to say the character went down hard. The rest of the team made it out, but in a rush and were not able to retrieve the fallen runner. As they had a hella time beating feet themselves and barely made it out.
Sounds grand and played like it should.
However, I screwed up my TNs for the perception test and shooting. (it has been 3+ years now) If i recall correctly, I slapped a standard +2 TN on the ultra vision. Where I believe I should have hit it with +1/2 the vis modifier, or +4.
Point being, only 2 of the guards should have seen him and as I recall only 3 bursts landed instead of 7. He would have been hurting and porbably not have gone down with all his combat pool and karma pool. It was a good character and I hated realzing how I Fed up.
But I didn't realize till the next day. (game went until 3am) The story was good but I feel I wronged the player. They did have a chance to recover him (on life support for info), but the cards were just not in it for them and the attempt was miserable.
Have you done something accidentally like this?
What would you do after realizing the error? (I hate to 'undo' a game, anygame)
(can you tell it is still eating at me... 3 years gone and still.

Edit: had - for TNs instead of + offhand I dont recall the exact screwup but it was definately a +2-4 TN oops.