The ‘forbidden riff’ is a guitar shop in-joke which began in Denmark Street, London, in about 1973. The joke that was originally seen in the movie Wayne’s World in 1992 was to make fun at Stairway to Heaven because it is one of the most overplayed songs in guitar stores when people are testing out new guitars. The guitar-shop workers were fed-up of hearing beginners ”butcher Stairway to Heaven”, ”Whole lotta love”, and other songs, quickly became “overplayed” to the point where you couldn’t walk into a guitar store without hearing someone playing it while they were testing out a new guitar. What’s worse, the people who came in to try out Stairway were well-known as the ones least likely to buy a guitar.
So they put up a list of ‘forbidden songs’, which included the aforementioned, plus some other classics.
If you look like you are going to buy an instrument, most guitar shops will let you play whatever you like. However, if you come in every week with your mates, try a few guitars and play one of the listed songs, they are likely to point to the list on the wall and tell you ‘time’s up’.