On Negation Tagging

I’m currently working on negation tagging to understand the sentiments behind movie reviews. To partially understand some of the sentiments, I’m negating words that occur after a negative word (eg. not, isn’t, wasn’t, etc). Can you answer the following:

At a college English lecture a professor said to the class, “Two negative words like ‘not bad’ can have a positive sentiment, but no two positive words can carry a negative sentiment”. A student in the back proved the professor wrong with two words.

What two words do you think he said?