Working on older or slightly neglected cars the nut bolts and screws especially those on the underside of the car that are exposed to the road spray and winter salt corrode easily.
A rusted screw head is not the end of the world the advice on this page will help help you remove a nut, bolt or screw with a stripped head.
First thing is not to panic, things are never as bad as they seem. OK sometimes things are as bad as they seem, if this is the case you may panic a little.
Lets imagine your using a 19mm socket to turn a 19mm bolt but due to the age of the bolt the metal has coroded and on your first turn you strip all the edges off the bolt. ACE.
The solution is so simple you will kick yourself for not thinking about it.
All you need to solve this problem is a file. File down the bolt taking away the croded metal and when you have a few straight edges uses a smaller socket or spanner to remove the nut. You can use the exact same trick with stripped screw heads.
The images below are from when I was changing the lower front suspension arms on a 7 year old Ford Ka, the heads of most of the screws stripped, oh they joy.