I couldn’t find a solution to this problem, so after a few tests, I found it myself.
Consider this scenario: an unordered list with floating LI elements inside (i.e. you build a slider). The LI elements have position: relative applied.
In this case, if you try overflow:hidden on the UL, it won’t take effect under IE7. What you need to do is wrap the UL in a div#ul-wrap:
div#ul-wrap {
overflow :hidden;
position :relative;
}