Postcode and zipcode verification
It seems WHMCS does not perform any verification checks on user address, specifically the Postcode/zip code (for USA addresses).
USA Postcode/zip code is either 5 digits or 5+4 with first value representing their state. You can find more details at http://en.wikipedia.org/wiki/ZIP_code
I'm sure there are also many already built codes available online that does the verification.
Not only is this a good feature to ensure accurate user profiles, it fixes a problem with Plesk integration.
Parallels Plesk Automation (PPA) performs postcode/zip code check for USA addresses when users are created. Since WHMCS does not check this, if a WHMCS client has improper postcode/zip code and has USA address, and WHMCS sends API call to PPA to create this user, it will fail because PPA will reject the invalid postcode/zip code.