Film tolerates overexposure very well; many photographers deliberately overexpose to ensure they'll get shadow detail.
Somewhere, on the net there's a web site where color film images were overexposed progressively up to about six stops and you could still get a decent print out of it.
Underexposure, however, is bad. You can't print what's not there.
Note that this is the opposite of digital, where overexposure blows the highlights out to max white, but underexposure still retains detail.