weird user problem

I am pretty sure that this is a bug but i have no idea how to replicate a problem. It happens at random times but it happened about four times now. When i load a page i have a cookie that remembers my user but at random times i will be logged in as a different user. Tonight when it loaded i logged in as Clyde or Cyde cannot recall exact user. When i refreshed a page i was back to my profile. Has anyone seen this before?