In SMP, if one or more people are wearing quantum boots, and someone else with quantum boots equipped logs on near them, everyone's jump height becomes 3 instead of 12-15. If you move far enough away (prob. chunk unload distance) your jump height comes back. There is no error in console client or server side. It will sometimes (but not always) fix itself after a while.
Issue known for quite some time and fixed ont he next release. About third time i say this.