Re: Why don't they crash?
As for 'unknown factors', if at your destination there is something that can substantially alter the course of a star, you're better off mistaking your course.
There's that, plus just run-of-the-mill uncertainty with the margin of error, that generally gets worse the more distant the destination.
“A life is like a garden. Perfect moments can be had, but not preserved, except in memory. LLAP” — Leonard Nimoy (1931-2015)