I got left behind ability-wise by a jazz band I was in just after I left school. They were just practicing and improving so much faster than I could manage, and they got in the drummer's mate who was at a much higher level than me at the time. Being told you're not good enough hurts, even if you have come to that realisation yourself. Guitarist has been pro for the ensuing 35 years and the sax player is really good for someone that doesn't do it as a job.
Got fired from a band with a school mate as they wanted me to spend more time networking at gigs in London with them, and was replaced by a younger, taller, trendy looking lad who was about the same ability on guitar. Did get to play with the singer again in a later incarnation though, so no hard feelings on this one.