When this sentence was presented to a usage panel, 74 percent said that the choice matters (writers aren’t free to pick the verb they prefer). Then, when this group was asked to name the correct verb, 78 percent selected happen and 22 percent happens (Morris 440). The majority opinion is right, but why did so many panelists pick the wrong verb?
Panelists who chose happens may have assumed that one was the verb’s subject. Educated writers have trained themselves to ignore prepositional phrases, such as of those things, when matching subjects with verbs (because subjects never appear in prepositional phrases). But in the sample sentence, one is not the subject of the verb in question; the subject is the relative pronoun that. That is singular or plural depending on the word it logically refers to, in this case things: Things happen [not happens].
Nearly all usage experts agree with this analysis, yet they have a hard time persuading some educated writers of its truth. Patricia O’Conner, knowing the amount of resistance she is likely to encounter, explains the rule quite carefully. She begins by showing when it is safe to ignore words in a prepositional phrase and when it is not. In the following example, which does not contain a who or a that clause, it is safe:

When the subject is who or that, it is not safe to ignore the prepositional phrase:

The subject of say is who, which logically refers to authors: Several authors say it best.
After giving an analysis similar to the preceding, O’Conner adds, “If you don’t trust me, just turn the sentences around in your mind and you’ll end up with the correct verbs: Of the authors who say it best, he is one. Of the authors, one says it best” (61).
Another way to illustrate the logic of the rule is to contrast the constructions one of those who and only one of those who, as in the following examples:

In both instances, who is the subject of the verb that follows it. In the first sentence, who refers to writers and is therefore plural: Several writers in the group have published a novel. In the second sentence, who refers to one and is therefore singular: Only one writer in the group has published a novel.
Conclusion: To choose the correct verb, you must consider the logical meaning of the sentence. Unfortunately, when you do choose the right verb, many educated readers will think—incorrectly—that you are wrong. If you want to play it safe, write around the problem: Helen and several other writers in our group have published novels.