Professor nitpicking is the common event of college professors using a fine tooth comb on assignments handed in by conservative students, while letting factual and logical errors on assignments by liberal students slide by. It is especially common at large universities, where there is less oversight on professor fairness and balance.
An example of this would be a professor ignoring a liberal's claim that New Deal policies singlehandedly ended the Great Depression, while harshly grading a conservative's assignment for a small (or nonexistent) error.