There can be little doubt about the existence of substrate effects in many cases when a whole population abandons their original language and adopts another. But there are situations in which the direction of linguistic influence remains unexplained, the causal connections are obscure, or the expected effect does not occur. One still does not know just how young children in American society manage not to acquire the foreign accent of their parents. If anything, the effect of parents’ language may be in the opposite direction from that predicted by contrastive analysis. Several cases of unmistakeable but inexplicable substrate effects are discussed: the initiation of the merger of /o/ and /oh/ by Slavic-speaking coal miners in Eastern Pennsylvania; the use of later for earlier in the English of Puerto Rican Spanish speakers, and the confusion of make and let among several generations of Italian-American speakers of English.