The Waiting Room is a stage of sorts in SSBB. It is played in when a person is waiting for a Wi-Fi match to load. The player is given the opportunity to practice for the match on Sandbag while they wait. The stage appears to be completely flat, with a pair of bottomless pits on each side, similar to Final Destination, likely due to its inevitable lack of true use. When Sandbag is thrown out of the stage, it reappears in the center. This "stage" is also used when testing new control schemes. It should also be noted that there is a "KO" zone for the top of the stage, although Sandbag cannot be killed by flying upwards.
This is because the Sandbag in the waiting room is the Sandbag item, only it doesn't release confetti or items. This also explains why it can't be grabbed, as well as why it can be eaten by Kirby, King Dedede, and Wario.