Hunspell. 17 (ever so slightly) weird facts about the most popular dictionary format.

(…) the dictionary format of Hunspell has a lot of peculiarities. Depending on your mindset, you might find the facts below curious, fascinating, ridiculous, or just plain boring. From my perspective, they are, first of all, didactic: they demonstrate the blossoming complexity of organically evolved software that solves the complicated task.

Relevant: Rebuilding the spellchecker.