As bad as "on accident" is, there's another linguistic tic even more nails-on-a-chalkboard-like: "agreeance." As in "The committee met for three hours in order to reach agreeance on the best course of action." Of course, I know what that sentence means. . . or meant. . .or MENT. (See what I did there?)
According to Dictionary.com, "agreeance" has a track record dating back to the 16th century. Now, however, the usage is "rare," and the word itself is described as a "bastardization of 'agreement.'" I'm no linguistic prescriptivist, but when one's choice of usage prompts the appearance of the word "bastard," I submit there is a problem.