69 (5e Variant Rule)
From D&D Wiki
This 'rule' is fairly meta in its use: Whenever you see the number 69 in the wild (videos, signs, games, etc.) the player (including DM) who saw such a thing must provide proof in form of a photo (screenshot, camera, etc.) and present it to be seen. 69's cannot come from the same source multiple times, they must be unique and time (how much is based on DM's discretion) must pass for it to be used again.
The player then must roll a D100 and the DM keeps track of all these numbers or 'Funnies'. The DM can expend a 'funny' during the session and the effects of this D100 can range in a huge variety. Such as a 77 roll suddenly turns a block of ice into a block of fire or a 16 roll makes a small dog become 6ft spontaneously.
DM's Discretion is king when determining the effects of the D100 rolls. The general rule of thumb is to make more impactful 'funny' events happen at higher rolls, though a DM can make their own list for this rule as a support or let their creativeness shine through for whatever they have in their mind to use in conjuction with this rule.
However, if you roll a 69 on the D100: The DM can do something of extreme importance or silliness: you can make a monunmental thing occur. Such as suprising your party with an encounter with the BBEG at a shop, make a dragon come out of nowhere in a hub city to terrorize it or send the party to another plane of existence. The possibilities are vast and endless for the DM and can be used spice things up significantly.