More than one person was involved. Why was? Doesn't more mean at least two? Yet even English scholars are wary of changing the verb to "were involved," even though we would say, "More were involved than one person."
Reference books do not offer much help with this conundrum, and the Internet is no help at all. But John B. Bremner's Words on Words and Theodore M. Bernstein's The Careful Writer both address the topic. Bremner sees more than as an adverbial phrase modifying the adjective one, "which is singular and therefore qualifies a singular noun, which takes a singular verb." That explanation might fly in the rarefied air of academia, but to accept it we must ignore the inconvenient fact that more than one person means "two or more persons," and would seem to require the plural verb were involved.
Bernstein doesn't try to justify More than one person was involved as good grammar—just "good idiom." He says "was involved" is an example of attraction, a linguistic term that accounts for certain incorrect word choices: "The verb is singular 'by attraction' to the one and to the subsequent noun [person]."
All but one ship was sunk is another example of "good idiom." The principles that apply to more than one also apply to all but one. If we separate all from but one, the verb becomes plural: Of the five ships, all were sunk but one.
One is free to endorse elaborate justifications for the validity of more than (or all but) one person was involved. But it is just as reasonable to conclude that this oddity is nothing more than institutionalized error—people have been saying it wrong for so long that we've become used to it, and more than one person were involved, the logical construction, sounds wrong. We see institutionalized error on the march today in untraditional usages like "each of them were here," "neither of you are right," and "a person should do their best," all of which we suspect will eventually be standard English, despite the anguished screams of purists.