It is easy to get caught up in the moment and write or do something that may seem funny at the time but remember that what you say can really hurt someone, or come back to haunt you. Think before you post. It only takes a second or two. Ask yourself if you really want to say it. Make sure you don't mind if your friends, classmates, housemates, lecturers, parents or employers hear about it later.
Remember that any information you post - whether in a comment, message, note, or a video chat - might be copied, pasted, and distributed in ways that you didn't intend. Before you post, ask yourself - would I be okay if this content was widely shared at university or with my future employer? How would I react if I saw this post on Facebook / Twitter?
At the same time, we all make mistakes. If you find yourself wishing you hadn't done something, delete it immediately and apologise to the person / people you upset. It is never too late to offer an apology.