What you say about yourself can give other people an impression that you might not intend. What you mean as a joke, may be taken seriously. I made the mistake of telling someone I had a bad memory. I find now when I forget something or have a momentary lapse it is almost like I get a nodding and a knowing look like, "Hmmm...we know she has a bad memory so therefore...". My innocent comment about memory has turned into something I never intended.
I used to work with someone who would constantly put herself down. “I am so dumb”, she would say. “How could I have done something so stupid?” It was hard not to get that impression about her, because she was always telling us how she felt about herself. As I got to know her I found she was none of those things, but was very intelligent and a good worker. She was not doing herself any favours by announcing her imagined shortcomings.
Joking at the office as well can be fun, but beware that you don’t start giving the impression that people can’t take you serious. You are fun to have around, but you would never be considered for something more expert or challenging. I mean you are the jokester right? I love laughing and I find it really helps relieve the stress of a busy day, but sometimes I have had to step back and evaluate what it was doing to my career. Am I giving people the right impression of me? I am a professional and at work that is the number one image I want to portray. My sense of humour is an asset to be sure, but it is not the thing I want people to remember when they are thinking of who to hire.
Sometimes we share with our co-workers things they really don't need to know. I would suggest that you keep your conversations with your co-workers on a different level. We don't have to be all business, but keep in mind that what we say about ourselves can make a lasting impression and when the time for promotions and evaluations come along, it might come back to haunt you.