I recently joined a few Stack Exchange sites on which I don't have the reduce ads privilege yet.
The issue I experienced happens with the two "leaderboard" style ads that are displayed to new users and visitors, like the following one (from a question on Super User):
The ad is loaded (probably using Ajax) shortly after the page itself is loaded (a few seconds). When it is loaded, it makes the content shift, which often will get users into accidentally clicking the ad instead of a link or voting.
A possible fix to this would be an immediate (no Ajax) server-side discrimination between privileged users and visitors, so that the server sends a fixed-height container for the ad to be loaded into.
That would prevent lots of inconvenience!