I used to get this issue now and then when calling out a mount, back during 2.x. It was fun to end up on an invisible mount. Have a few screenshots of me sitting astride Amalj'aa tails or some random inanimate object. Experienced it once or twice with some equipment pieces back during 2.5-2.55 when the gap between chestplate and trousers went missing, leaving me with a cut-off midriff.

I guess it's similar to when your internet connection glitches out while loading a webpage, ending up with some of the css or images not loading properly, and you have to refresh the page (sometimes several times) to get it to work. Haven't seen it lately, though. So like Bracken said, it could be a dx9 issue.