Someone >_>: My code works and I know why
Someone else >_>: My code works and I don't know why
Me >_>: My code doesn't work and I don't know why

The scariest scenario is: My code is working and I don't know why

Then come the hours of stepping through your own code and cursing just about everyone along the way, from library authors for a not good enough documentation, through kernel developers all the way to cpu manufacturers...

Only to find out you forgot a semicolon at line 248...

Know the pain.