When Grimm's law was discovered, a strange irregularity was spotted in its operation. The Proto-Indo-European (PIE) voiceless stops *p, *t and *k should have – according to Grimm's law – changed into Proto-Germanic (PGmc) *f (bilabial fricative [ɸ]), *þ (dental fricative [θ]) and *h (velar fricative [x]). That was known to be the usual development. However, there appeared to be a large set of words in which the agreement of Latin, Greek, Sanskrit, Baltic, Slavic etc. guaranteed PIE *p, *t or *k, and yet the Germanic reflex was voiced (*b, *d or *g).
At first, irregularities did not cause concern for scholars since there were many examples of the regular outcome. Increasingly, however, it became the ambition of linguists like the Neogrammarians to formulate general and exceptionless rules of sound change that would account for all the data (or as close to all the data as possible), not merely for a well-behaved subset of it.
One classic example of PIE *t → PGmc *d is the word for 'father'. PIE *ph₂tḗr (here, the macron marks vowel length) → PGmc *fadēr (instead of expected *faþēr). The structurally similar family term bʰréh₂tēr 'brother' did indeed develop as predicted by Grimm's Law (Gmc. *brōþēr). Even more curiously, they often found both *þ and *d as reflexes of PIE *t in different forms of one and the same root, e.g. *werþaną 'to turn', preterite third-person singular *warþ 'he turned', but preterite third-person plural *wurdun and past participle *wurdanaz.