fio

fio is listed here as its own lemma, and it is treated that way in every dictionary, but the honest description is that it SUPPLIES THE PASSIVE OF facio. facio has no passive of its own in the present system, so where you want "is made, is being done", Latin uses fio; and where you want the perfect, "was made, has become", it uses factus sum, the perfect passive of facio. One meaning, two stems, and that split is genuinely confusing until somebody says it plainly.

The forms are odd in the other direction too. fio, fis, fit, fimus, fitis, fiunt look active, but the infinitive is fieri, which looks passive, and the imperfect is fiebam. The subjunctive fiat gives fiat lux and fiat voluntas tua. Alongside the passive job it works as an ordinary intransitive verb, "to become" with a complement (dives factus est) and "to happen" on its own (quid fit?). Note that only the simple verb behaves like this: the compounds of facio form a normal passive, so it is conficitur, not a fio form.