The following sentence can be found (as of 15 September 2016) in this Wikipedia article about the effects of rape on the victim:
Sometimes in an effort to shield oneself from believing such a thing could happen to their loved one, a supporter will make excuses for why the event occurred.
The clash in pronoun choice (the switch from one to their) makes this clearly anomalous. What exactly could have led to its being written? I think at least two unease-promoting factors are involved.
First, although everyone knows the indefinite-reference third-person pronoun one, and its inflectional forms one, one's, and oneself, it sounds rather pompous, and one is somewhat reluctant to use it. As soon as one gets into constructing a sentence that requires one to use multiple tokens of one, one finds oneself feeling reluctant to proceed: one is trapped in a linguistic mode that makes one sound as if one is a member of the British royal family, which of course one isn't.
Second, although indefinite-reference they with syntactically singular antecedents (No one should insult their mother) is perfectly natural and idiomatic, we balk at using its reflexive form because we aren't quite sure whether it should be themselves (as with syntactically plural antecedents, when it denotes a group of entities in the ordinary way) or themself (which certainly exists and has been used for hundreds of years but is currently rather rare and might be regarded as non-standard).
So the hapless writer must have started writing this sentence:
And promptly found that two occurrences of pronominal one in a row followed by loved one (which contains the common noun one, a different item) was one one too much for one.
The alternative would have been to try this:
That not only has the potentially deprecated singular reflexive themself, which broadly I would favor, it uses it cataphorically: the reflexive pronoun comes before its antecedent (their), which itself is a cataphoric pronoun, preceding its own antecedent (a supporter). That all makes the use of singular they rather salient, and may have given the writer pause. And of course it is no better to write this instead:
That has the same problem, plus another one: the cataphoric themselves clearly sets the reader up to expect a plural antecedent, because of its visibly plural morphological form, but then there turns out not to be one, and the singular ultimate antecedent. a supporter comes as rather a shock.
What is left as an alternative? The appallingly clumsy disjunctive he or she would be one possibility:
That is so grotesquely clumsy that I don't think any writer of conscience could stomach it.
Writing English prose isn't easy, is it? I leave comments open below to allow readers to experiment with rewriting the sentence so that it is smooth and idiomatic and not susceptible to prescriptive quibbles or style criticism.