In fact your certificate should say Airplane Multiengine Land (VFR Only). Your chances of getting caught are slim; however, if you were to be involved in an accident or incident, and someone started looking, then it could get ugly. Especially if you were actually operating a multiengine airplane under IFR. Since your current certificate is not a valid one, anything that you do in the future could also be negated.
I would recommend that you contact your nearest FSDO, and present your certificate, logbook, training records, etc. They will make the appropriate correction. So long as you haven't operated under IFR in a multiengine airplane, there will be no repercussions to you at all. Just say something like, "I just last week noticed that ..."