Nope, not the Y-men or retcon stuff, but the Welsh language. Almost everything in Welsh inflects depending on place and role in the sentence. The one that caught out Jenny 1938 was the soft mutation. "Rwyt ti'n ferch?" "Merch o hyd"
The "girl" in the first example follows the predicative, "yn", and so has a treiglad meddal, a soft mutation. The same word in the next doesn't, so stays as the radical,or root form. The treigladau are cysefin, meddal, trwynol and llaes, which in English are radical, soft, nasal and spirant. Thus, for various aspects of Wales, you have Cymru, i Gymru, yng Nghymru and a Chymru. The first line of the national anthem "Mae hen wlad fy nhadau yn annwyl i mi" contains one soft "[g]wlad" because it follows a preceding adjective, and one nasal "[tadau]nhadau" because it follows the possessive.

Prepositons inflect as well, just to really make your heads hurt.

Angharad mentions the verb "bod" to be. There are two ways of saying something in Welsh. You can inflect the verb-noun, like Latin, or you can use an auxiliary verb. South and West Wales use "bod", but the North do odd things with another, daru. The sentence structure in Welsh usually reads "It is that I am in the process of doing something" or "bod + pronoun or subject + predicative + verbnooun + object"
Mae eira [y]'n dod---snow is coming
Mae eira wedi dod---snow has come