The Smack Wall Sweeper was an obstacle that featured in Season 3 of Wipeout. It featured a beam and two four armed sweepers that the contestant must avoid. However, there is also a wall next to them. As the contestant runs along the beam, part of the top half of the wall will flap out and smack the contestant in the face. The contestants aren't told anything about the wall, so they don't know that it is coming.
The Smack Wall has since made many different appearances in obstacles. It's next appearence was in Winter Wipeout, in Nana's House. Again, the Smack Wall smacked out and hit the contestant, but it followed a mechanical pie in the face and a hydraulic dog named Fido. It also appeared at the Gingerbread House- Nana's House decorated in sweets. It has also appeared in Spring Wipeout, at the Bed Bugs. This time, it took the form of a headboard on a bed.
Later in Season 4 it was again the same as in Season 3, the letter P being the first half of the Wipeout Wall but now it is joined by a letter U being identical to the Fence Flappers.
In Season 5, had the Smack Wall with the same features but the sweepers being saw blades in the Buzz Saw.