Yes and it makes me grouchy.
I've had stuff break or mechanical problems that I couldn't figure out how to fix on the road. I've gotten so over-cooked that I was dizzy and light headed and didn't think that it was smart to continue. I've even ended a couple of rides in the back of an ambulance with broken bones.
There's always new things to learn. After breaking the seat binder bolt a couple of times I started carrying a spare on our tandem. The next time, however, the whole seat clamp broke so I started carrying one of them. Rider physical issues are harder to plan for and, one of the symptoms or heat sickness is impared judgement. There's a lot of wisdom in knowing when to soldier on and when to cut your losses.