Although I ALWAYS order my plain hamburger WITHOUT cheese, Wendy's regularly puts chesses on it. This is why I always inspect my burger before I leave the drive-thru.

I hate to be the one to tell you this, but fast food employees are generally very poor at paying attention to customers.

Since they cannot tell the difference between a plain hamburger (just a hamburger and a bun; NOTHING ELSE) and a plain CHEESEburger (just a hamburger, bun and cheese ONLY), I'd advise you to ALWAYS check your burger before consuming or leaving the business.