When I was travelling and using dodgy internet cafes I used to see this happening sometimes (really quite rare). I appeared that the downloaded page was some how corrupted, and the hidden items on the page became visible and fixed into a location (at the top of the page). A refresh usually fixed the very intermittent problem.
I shouldn't worry about it too much though. The same used to happen (and more often) with hotmail.
And another thing, it happened on PCs.