Y-H-B was ordering his evening meal at one of his favorite restaurants in Manchester, Vermont, the Marsh Tavern at the Equinox Resort, when the waitress taking his order, a young woman––and evidently a native speaker of American English––used the form “teached” instead of the correct past tense form “taught.” From the rest of her utterances I gathered that she was at least a high-school graduate, so the use of a completely wrong past tense was totally unexpected.
Theorists of language change (with the notable exception of Henning Andersen, Roman Jakobson’s most outstanding Harvard student) barely if ever mention speech errors as drivers of change, although such errors must to be taken into account, especially when they are the product of imperfect learning and false analogy. The waitress clearly applied the normal rule for the formation of the past tense of typical weak verbs (like “cover/covered”) to a strong verb, which is what yielded the incorrect “teached” instead of the correct “taught.” But that is exactly how languages change, i. e., when errors are made and then propagated by other speakers equally inclined to violate the rules of grammar as was the initiator.
Language change is not always predictable or regular. It may be the product of individual propensities, including the creation and propagation of errors.