Feels shit, doesn't it?

I left my job yesterday. Depressingly pretty much the last thing I had to do was to file a report to my line manager on how a colleague of mine had spoken to a vulnerable client the previous day. She was absolutely in the wrong and I don't think I had a choice but to pass on what I saw (especially as this woman is supposed to take over from me in working with this client and lots of other client she's supposed to be taking over from me also saw the incident and are really concerned about working with her now) but I feel pretty bad about it.

Have you ever got someone into trouble at work? How did it pan out?