Oh I have a good one. This ingrained the practice of asking one simple question "What has changed recently?" and the practice of reevaluating the situation to avoid going "Down the rabbit hole"
This was many years ago. I was working as an on-site IT Tech going to various homes & businesses throughout the day. I arrived on-site and fixed their issues. As a good measure I went ahead and updated and cleaned temps . Lo and behold, the computer booted to a black screen with blinking cursor. I did EVERYTHING, and I mean EVERYTHING to attempt to repair the boot. Bootsector repair, chkdsk /r , + various other fixes. I was still green in some ways and did not stop and step back to reevaluate the situation. I spent 2-3 hours attempting to fix the boot on this lady's laptop with no success. Eventually I figured it out. The Hard Drive had been upgraded recently to a 320GB. Unfortunately, the laptop only had 28 / 32 bit LBA (forget which one is applicable) and the previous tech had installed this large drive without paying attention to this limitation. So once the windows update wrote data above the 137GB mark, the drive was not bootable. I partitioned the drive into 120GB partitions and defragged and voila! It started booting again