A Wikipedia page loads. The last thing to load is the banner. This pushes the page content down. If you've clicked on a link near the top of the page, the banner grabs it instead.
This happened last year and it was reported then (and it was incredibly annoying then too). It also fouls up stats on banner effectiveness, as banners are clicked on without intention to do so.
Please load the page with a space for the banner to avoid this effect.
- d.