In a clip from a news conference held in Sweden that has been replayed many times over the radio in the last two days, Barack Obama is heard uttering the following sentence: “The use of chemical weapons are abhorrent.” This sentence contains a flat-out grammatical error, since the subject of the sentence use is in the singular, whereas the verb are is the plural of the copula be. Obama is undeniably a native speaker of American English, and he nevertheless uttered this sentence without correcting himself.
Native speakers, let alone foreigners, make mistakes when speaking their own language. The gamut of errors is fairly broad, ranging from simple slips of the tongue to the most egregious grammatical errors of the sort just instanced by Obama. A number of causes for error come to mind, all of which come under the compass of linguistic competence, including the stress of the moment, memory lapses, etc. But is there such a thing as a logic of linguistic error? The question remains open, although in the case of a failure to coordinate grammatical number between subject and verb, one could appeal to the force of assimilation (however weak), since constructions involving a collective noun in the singular governing substantives in the plural is idiomatically aligned with plural number in the verb, e. g. a bunch of guys are standing in line, etc.
All the same, a grammatical error that goes uncorrected always raises the question of cognitive competence in general, not just one that pertains strictly to the utterer’s command of language.