"Why didn't find find my file?" I wondered sometimes. "I know it's there!"
More often than not, I'd forgotten to use -print. Without -print (or -ls, on versions of find that have it), find may not print any pathnames. For a long time, this quirk of find confused new users, so most modern versions of find will assume -print if you don't supply an action; some will give you an error message instead. If you don't get the output you expected from find, check to make sure that you specified the action you meant.
—JP and DJPH