[Oddball sees that the bridge he wants to cross is intact and is pleased with himself]
Oddball: It’s still up!
[a plane flies over the bridge and bombs it… direct hit]
Oddball: No it ain’t.

— Kelly’s Heroes

Don’t count your chickens before they’re hatched.  In my life, it’s tended to be “Don’t burn your bridges before you cross them.”  Keep your eyes on what’s in front of you, watch for things that will change your conditions, and always be flexible when it’s time to plan and re-plan.