The most unpredictable Google OneBox is definitely the image OneBox. For some queries that are popular in Google Image Search, you'll three image results at the top or at the bottom of the page. In most cases, the images are relevant or at least innocuous, but if you search for Google logo one of the image results is a little bit inappropriate, as Search Engine Land noticed.
In March, Search Engine Roundtable reported an explicit image result displayed when searching for [hot celebrities]. Instead of writing a blog post to complain or sending an email to Matt Cutts, you can now report the offensive images using the small link displayed above the image results.
To disable the image OneBox, you can select "strict filtering" for SafeSearch in Google's preferences, but this also filters web pages that contain explicit text.