Mobile API: What could possibly cause one logged-in user to post as another user?

[PleaseHelp]

We are completely dumb-founded.

For about 30 minutes today, one logged-in user was posting as another completely different user via the API. Now that user is back to posting as themselves. They are two completely different users (we've already double-checked), so this is not a case of the same user having two accounts.

We don't even know where to begin to investigate this. This only happened once, but it's incredibly strange because those two users are completely different. It's almost as if their sessions got mixed up or something, but how is that even possible?

You can imagine the confusion this caused in the forum when that user was posting and their posts were coming up under a different username.

What could possibly cause one logged-in user to post as another user via the Mobile API ? Where should we be looking to investigate this?

Thanks.