Corrected entry: When we see John's office, he is listed on the door as John Smith with Professional Engineer below it. A Professional Engineer (P.E.) is not a type of engineer but a professional group designation. While it would likely be on the door, so would the actual type of engineer that he is supposed to be. His door should read John Smith, P.E. with the type of engineer (such as Civil Engineer or Construction Engineer) below it.
Xofer
25th Jun 2005
Correction: Given that John isn't actually an engineer, and that the office was a front, it's more of a character mistake (his forgetting to add the type of engineer on the sign) rather than a movie mistake.
Xofer