Van Halen’s pre-show instructions included the famous “no brown M&Ms” clause to give them a signal that the show runners read the technical requirements.
Van Halen were known as divas in the rocker world. One famous piece of anecdotal evidence was their contract for them playing concerns included a clause that the backstage area needs to have a bowl of M&Ms, and moreover that bowl of M&Ms must have all brown M&Ms removed before their arrival.
What they were actually doing was giving themselves a signal to determine if the concert showrunners had actually read and followed through on the contract and the instructions for preparations for the show. The M&M clause was mixed in with a lot of other technical & logistical linchpins that were required for Van Halen’s large, pyrotechnic shows. Most concern showrunners would need the extra instructions, all would have claimed to have read it. The brown M&Ms was a big of Manufactured Feedback the band could use to see if they needed to have someone double-check the rest of the work performed in advance of the show.
Source
I don’t remember where I first learned this.