I've been fairly lucky in that either it was fairly obvious that the detour was the way to go and I saw it before it was too late or I was able to cross in some way. I have waded across shallow water a few times and walked a narrow railroad trestle once.
We have hit a long stretch of broken up concrete that was nearly unrideable for too many miles of construction. If I remember correctly they shuttled bikes around, but weren't doin so for the weekend. There was one young kid left on the site. We asked if there was a way around. The directions went something like, "Go back 75 miles the way you came...". So we slogged over many miles of broken concrete of the closed road. I don't recall exactly how far it was, bur it seemed interminal.