Legally, as long as they not alleging that you laughed at them due to them being in a protected category (e.g. you didn't laught at them due to their race, religion, age over 40, sex, disability, etc.), legally, you did nothing wrong. That's the good news. (If you did laugh at them due to them being in a protected category, then you may have committed illegal workplace harassment and the employer may be required to take action against you to avoid their own liability.) But the bad news is that if you don't have a written employment contract limiting the reason or process by which you could be disciplined or terminated, you are an "employee at will." That means that if your employer voluntarily, of its own free will, chose to take action against you, up and to including termination, they could; doing so would be legal. As to whether you should contact HR: that depends on corporate culture, the personalities in your HR department, etc. There is no right or wrong answer: it's what you feel, as someone who knows the company and HR, would be best.