I almost left my last job without anyone trained to replace me and I was the bottleneck every order in the company went through (despite being the lowest-paid employee). Just by chance, they finally hired someone part-time to take some weight off of my shoulders a month before I went on FMLA medical leave and I ended up having to quit my job while on leave. They were very close to being very screwed because the process was complex and they had just finished with a new back end of the website that was also very complex and only I knew how to do my part.

I kind of wish I had the satisfaction of leaving them totally fucked because I hated that job.