Vulgar Latin to modern Romance
This is very likely well known to you. The emergence of the phoneme /ʒ/ written "j", from /j/.
Classical Latin ego /ˈe.ɡo/, probably elided the middle /g/ to form Vulgar Latin *eo, which must have been pronouced something like /jo/, yielding modern Sardinian eo /ˈɛo/, Spanish yo /jo/, Italian io /ˈi.o/, but Central Catalan and some instances of Old French jo /ʒɔ/, modern French je /ʒə/.
The effect is much stronger with the /j/ at the beginning of Latin words. See for example iocus, which underwent fortition in most varieties of Romance.
Middle Chinese loans in (Northern) Vietnamese
See this StackExchange answer about certain Middle Chinese lexemes in /j/ (preserved in modern Mandarin and Cantonese), which surface as voiced fricatives /z/ in northern Vietnamese, written as d (not to be confused with đ) in modern Vietnamese.
Holtzmann's Law / Verschärfung in Proto-Germanic to Old Norse & Gothic
E.g. Proto-Germanic *Frijjō resulting in Old Norse Frigg, with a /g/ that is retained in modern Scandinavian varieties.
EDIT: Index Diachronica has a list of phonemic changes. For a lot of them I haven't been able to access the original source, but the list is quite long.