Jump the gun
Meaning: to do something too soon, before you have thought about it carefully, to start before the starting signal. (Originally used in sports contests that are started by firing a gun.)
Example: We all had to start the race again because Jane jumped the gun. When we took the test, Tom jumped the gun and started early.
Example: We all had to start the race again because Jane jumped the gun. When we took the test, Tom jumped the gun and started early.
