I was looking at the web site http://librarytypos.blogspot.com/ and found this entry about mountain being misspelled on a sign in Colorado. This may have been an April Fools' Day joke. I'm not sure.
Just out of curiosity I did a keyword search for mountian in our online catalog and found only one entry. It is in a contents note AND it was not in a record with HCPL holdings but a record from another library in our consortium.