Then you know it is crash time. Unfortunately with 8.5.1 FP1 this happens quite often (on one of my machines).
I know I should not care about Lotusscript anymore because IBM says it is dead. Unfortunately if you have 15 years of code, sometimes you just cannot move on (except you migrate and even it that case you sometimes need old code).
The good thing is. If you recognize that Designer is going to die in a second you can at least close and save all other open libraries. Just leave the "faulty" library as it is. Designer will crash if you close them. Still I find this very annoying.Today's crash statistics
IBM Domino Designer: 13 (then I gave up)
Netbeans 7 Beta: 0
Who says IBM can't win anymore?