- Memetic Mutation: How the ads became notorious. So much, there have been various spoofs such as RockOn, a "Blooper" video featuring someone with no hand (and a floating head) among other things, Gmod videos advertising Headcrabs, and even Disaster Movie got in on the HeadOn spoof.
- Sequel Displacement: Believe it or not, "apply directly to the forehead" was not the first slogan for HeadOn. The first ad used the phrase "Should I know about HeadOn?" and a transitional variant used the "apply directly to the forehead" slogan only once.
YMMV / HeadOn