To illustrate what I'm referring to, I'd like to give to examples in English:
While must not seems to be the opposite of must, it has a different meaning. Must go to school, for example, means I have to go to school, while must not go to school indicates that I am not allowed.
Another similar negation which is not truly the opposite is about to and not about to. When I say I'm about to leave I am gonna do soon. But saying I'm not about to means they I definitely won't do it. So, it's not quite the opposite.
If someone is not aware of these differences, he certainly will use them incorrectly.
Q: I wonder if there are similar examples in Spanish, which language learners will run into if they don't know.