It's pretty well marked, but it's also 800+ miles through diverse terrain, you'll likely get off course once in a while. It also depends on how well prepared you are with maps, electronic navigation aids, etc. And how engaged you stay-are you anticipating the next turn, or day-dreaming; fresh and aware, or tired and zoned out? It ain't a walk in the park, some areas are extremely remote where it's only you, the critters and a few other hikers now and then. The most common "lost the trail" scenario is where the single track takes off from a two-track. Note that not all that long ago people followed shiny tags hung in the bushes for a long section. If you use good maps, keep your head up and are somewhat experienced, I would expect getting lost to be pretty low risk.
The bear went over the mountain to see what he could see.