Using a comma before "but"

I was once told by an English professor that a comma should never be used before but in a sentence. For years, I have followed her advice but sometimes I just feel like it just needs to be there. On the other hand, my brain also tells me that the word but is, in a sense, performing the same function as a comma in the sentence.

Does anyone know if there is a rule for this?

Larry Trask's Penguin Guide to Punctuation makes it clear that it is permissible to use a comma before 'but'. The OWL Perdue Writing lab does the same. The BBC Learning English site likewise.

You were misinformed.