- Look before you leap. - Measure twice, cut once. - Stay silent and have people think you're a fool, open your mouth and confirm it.
I'm sure there are many more common idioms pointing towards the idea of "think before you act".
- Don't dive head first into water.
- Measure nine times, cut once.
And a random saying I just remembered: "Autumn has the eyes of a summer and the teeth of a wolf."
Edit:formatting
- being late (acting in "autumn") ...looks/feels similar to
- being on time (seizing the moment at the right point, in "summer")
...but has a nasty bite?