Likewise, Miss Amadine. Likewise. "They" and "themselves" are pronouns referring to multiple persons, and "one" is clearly not a multiple number. Your grammar is atrocious.
I think they're wrong....but then I got to thinking.
This SHOULD be:
I thing they're wrong...but then I got to thinking:
I think they're wrong...but then I started thinking:
I think they're wrong, but I started thinking:
My bold: When one is going to correct other's posts, one should check what it is that they themselves are posting first...
Again, you are wrong on a second and plausibly a third account, Miss Mmeeshal. "They" can be used in both the singular and plural numerical, but, in the curious instance of The Honourable Lady Amadine's example, "they" can be used as the third person singular as an indefinite pronoun where the logical gender is unknown. As she used "one" in the singular, "they" is acceptable in the same sense. This is a tradition which dates back to the fifteenth century and, while I am lazy and detestable and cannot be bothered to provide specific instances, I will not have you ride rough-shod over our fine mother tongue. In the likely case further clarification is required, I refer you to that authoritative and contradictory work Wikipedia
Regarding the plausibly third error, "themselves" is, in fact, used correspondingly as the third person plural either emphatically, reflexively, plurally, dialectally, or after-a-prepositionally. The correct (or, as we like to say, what sounds most sophisticated is plausibly the most correct if only for our own egotistical edification) way for THL Amadine to have used themself/themselves would have been in the third person singular emphatic: they themself (though the non-standard "they themselves" is equally acceptable in some dialects.) Neither you nor The Honourable Lady Amadine can be faulted for assuming the positions taken.
For clarity and cheerful hobsnobbery, an example has been generously provided below.
"When one must correct another's post, one should check what it is that one oneself will post first."
It is regrettable that such a fierce tone must be employed, but, as they say, dulce et decorum est pro matria lingua mori. And remember, the passive voice must not be used. Tally-ho! Excelsior! WHY IS ALL THE RUM GONE?!