Here's a really bad one:
a customer had only Dev and Prod, no Q-system.
Support package levels in both were different.
THe customer urgently wanted a correction (SAP note with guidance for manual changes) in production. I told them they need to get the support package in Prod first, but they said they couldn't wait. and pushed.
So, I thought: well, if it goes wrong, I can always turn things back again and get it back to its former state with a new transport
Unfortunately, there was a function module involved, so the transport would pick up the whole function group. And it had a syntax error in Prod due to the missing support package. But this function group was involved in all communication the system did. You couldn't even log on - it created a syntax error. I tried to transport a change to take it back, but the RfC call from Dev to Prod also ended with a syntax error. the system was, for all intents and purposes autistic.
Panic!
I suggested we need SAP support to go in on database level to sort it out, but customer IT said, they can get a backup in exactly to one minute before the crash. It worked. luckily the downtime was late Friday afternoon and there was no ESS etc those days, so no complaints.
It turned out later, when another customer ended up in the same situation (without my involvement!) that SAP support could indeed have sorted it on database level, but the backup was the better solution.
BIg lesson learned. Ever since, I've pushed back very strongly, when a customer asked me to do something I knew was too dangerous. Luckily, I was ca. 13 years younger back then - today, such a rush of adrenaline would probably give me an immediate heart attack ;-)