Not to be taken literally, this refers to a player crafted build during a run (or a combination used in such a build) that is so successful that it “breaks” the game and there is no challenge possible for the rest of the run. Likely the build is killing enemies so quickly or healing so successfully it’s simply not possible for it to be killed before it has gained enough XP to (often through an auto level option once you’re at a certain point) to keep ahead of the curve. This is seen as a major goal – to discover themselves and make a broken build in the game – of many survivors-likes players. See also: Invincible.