I know I have seen this discussed before (it's been a while since I've been on) but I have hot water on the cold water side in the bathroom sink. It only lasts for a few seconds but hate to waste the water. Thanks
Could that be because the cold water line is too close to the hot water line or another source of heat? I have seen this before. Separating the lines or placing insulating material between two lines may help.
If this is single lever faucet then it may to be re-built. As water from one side leaks over to the other side when not in use. My engineer son-in-law could tell you the scientific reason for hot water moving into the cold water. I just know it happens.