Yes, I've definitely experienced that quite often -- despite having been exposed to Ian Stewart's good advice in Letters To A Young Mathematician that your first tactic when you don't understand something should be to just keep reading.

IIRC, Feynman once said "read until you can't understand anything, and then start from the beginning"

Yeah, sometimes, though I remember more cases where I was "wait a minute, there's a mistake here!" or "this is completely sub-optimal" and would put that in my notes, and a few paragraphs later he would explain the mistake or how to do it more optimally etc. But I don't see it as wasted effort, more as a small opportunity to pat myself on the back at how well I understand the material - I guess it's the difference between thinking about it five minutes versus half an hour.

This is why I read through at least the whole chapter first, and mark paragraphs I don't understand along the way. Annoyingly often paragraphs are just poorly written, and make sense only when you've learned the whole concept elsewhere.