According to this page from BuddhaNet, it is mentioned that there are three things Buddha cannot do. They are not mentioned in there and I don't know how reliable the source is.
- The Buddha is omniscient but he is not omnipotent. He is capable of innumerable feats but there are three things he cannot do. Also, a Buddha does not claim to be a creator of lives or the Universe.
I just found this question discussed on Dhamma Wheel. However, most of the replies are opinion-based.