As a Usenet discussion grows longer, the probability of a comparison involving Nazis or Hitler approaches one.
Attributed to Mike Godwin, originally referencing the Usenet, now generally applied to any internet forum. (wikipedia)
In layman’s terms, the longer a thread is, the more likely someone will compare their opponent to Hitler. It is an association fallacy or ‘guilt by association’. Making the comparison typically reduces the discussion to other colorful comparisons, often ending in name calling and questioning of sexual preference.