MavenBadlyFormedProjectErrors
This page lists the various errors you might get when opening Maven projects in NetBeans. Each page discusses in detail how to get rid of the problem. The error names as listed here appear in the 6.7 release. Older versions might have different or more generic wording, but the root cause is often the same.
If you feel everything is ok, but you still see an error, you can try to scratch the local Maven repository (under $HOME/.m2/repository, preferably just the relevant part of the repo) and relaunch a build (the repository will be recreated). Sometimes this fixes the problem.
