This one is especially difficult when it happens in the usual scenario (a guy does something weird and his girlfriend has to lie about his behavior, although this does happen in reverse as well). This can be a complicated situation as both partners may not be willing to talk about it, and one of the partners might not even be completely aware of what is happening. But, if you are lying about your partner's behavior, it is most probable that you are going to be the one to do something to fix this.
First of all, you are going to have to talk to your partner, but this is not going to be an easy conversation. You are going to have to really open up, and talk things through. You are also going to have to explain to your partner why the behavior they are doing is bad or weird in the first place, as most probably they are not even aware of this fact.