Karl Verner identified the principle that they instead become the voiced consonants *b, *d, *g, *gʷ if they were word-internal and immediately preceded by an unaccented vowel in PIE.
In East and North Germanic, the levelling was almost complete before the earliest records, but Gothic and Old Norse had traces of grammatischer Wechsel.
German also features d-t in leiden, litt, gelitten ("to suffer") and schneiden, schnitt, geschnitten ("to cut").
Apart from the English copula mentioned above, the only occurrences of s-r in the modern languages are in Dutch: verliezen, verloor, verloren ("to lose") and verkiezen, verkoos, verkoren ("to choose").
Likewise, the terminal devoicing that produces a fortis-lenis alternation in Dutch (wrijven:wreef) is an unrelated historical phenomenon.
Examples are numerous in the older languages but are less frequent today, because some levelling has occurred, and in some cases, one verb or the other was lost.
heffen ("to lift/raise", from the strong verb *habjaną) - hebben ("to have", from the weak verb *habjaną) The term Grammatischer Wechsel was originally applied to any pair of etymologically-related words that had different accent placement, including also Proto-Indo-European athematic nouns.