My worst 'bad nitrous', for a half a year due to trauma caused by dying kittens ón me in terrible ways doing nitrous oxide while on LSD would occasionally lead to very bad trips where I mistakinly took the fast onset of the dissociative for 'realising something is dead', which caused horrible mindfucks. After the trauma was dealt with it never happened again and it hasn't happened for a good year now.
Either way, the worst one was that I thought I realised my friends had been dead for hours and I was just partying and chilling with them and was somehow so very insane I only just realised that moment they had been dead for hours, and that I was a really messed up person for chilling with dead people for hours without noticing it. That was probably the worst drug-induced experience I've had. It wore off as soon as the nitrous wore off, bad nitrous moments would never turn my trip bad, LSD itself is always good fun.
Being dumped while on a 4 hit LSD trip on a festival in a foreign country where I was with no other friend then my boyfriend, who went batshit insane in the peak and dumped me and then ren off the be gone for about 4 hours wasn't fun either, but not as horrible as it should've been, I didn't cry, didn't freak out, didn't get scared or anything, I was just pissed off and confused and it wasn't fun to be lost alone at night on a festival where I knew no one and was tripping really hard, and thanks to the overly intense visuals I had a really hard time trying to find him again as people would just appear out of visuals or become invisible due to visuals, or change shape into different people altogether, couldn't tell where he was at all. Not great, but probably not the worst, probably the worst thing that happened while on drugs though.
Also seeing a friend having an epileptic attack on magic mushrooms wasn't wonderful either, again not horrible, just unpleasant.
Other then that no bad shit ever happened to me.