One day, an inexperienced user posted a personal anecdote to the front page, in which a cashier at her supermarket was unable to identify a portobello mushroom. The only link in the post was to a .jpg of a mushroom, and the poster then took ten minutes to craft a "more inside." Several other users proceeded to tease the original poster about the lack of quality and the amount of editorializing in the post, and she became more and more defensive. The thread quickly degenerated into jokes about mushrooms and increasingly agitated remarks from the original poster, who was mocked for her apparent snobbery and unfamiliarity with the conventions of MetaFilter.
Somehow, Matt failed to notice the thread until it had generated well over 200 comments. When it finally gained his attention, he thought it was funny; as a result, it was not deleted, and continued to collect inane comments and inline images even after the drama had dissipated and the original poster apologized for the fracas. The thread ultimately generated a total of 1,032 comments before being archived. Jokes about mushrooms and references to "the mushroom thread" persist around the rest of the site.
Sadly, it has been reported that the original poster of the mushroom thread died in February, 2006.