Really, if an employee has a legit issue, he should go to HR and take care of it that way and share his answer with his coworkers. Even if he is in the right. The workgroup meeting is never the right place to make a stand. It's rare that there's a benefit to it.
Nine times out of ten an employee thinks he knows a policy or all of the facts that surround a certain circumstance and, oh brother, he's going there. The manager has 2 options. One is to do nothing, look weak, and allow himself to be undermined by an ignorant/misinformed/belligerent employee. The other is to correct the employee in front of everyone, but that can discourage others from asking appropriate questions later on.
There's also a 3rd option, "There is additional information that applies to that situation and I'll be happy to discuss that with you later," but that doesn't work often because the loudmouths don't know when to shut up.
