That happened to me too! But I assumed that I had either clamped the bike in incorrectly and it must surely be leaning or that the front wheel was out of true to an alarming degree. I hopped off and determined that the front wheel spun just as it should and that the bike was indeed clamped in correctly. I just assumed that the effect was just some sort of optical illusion (having recently done some reading in philosophy of the "senses can be deceiving" variety this seemed reasonable...). This means there is something wrong with me now?!