I know there are posters that tweak details to make the situation less recognisable. They'll change the names of those involved, the genders, their relationships to them, names of organisations, locations, amongst other things. Sometimes they get confused with which details they've switched.
While I don't do it, I expect a certain degree of fabrication in any post. I don't think it makes a difference.
I think it's rude to call someone out on these discrepancies for no reason other than to go "liar!" Unless the specific details are relevant to the answer that would be given. In that case I think that it's acceptable to ask for clarification, but do it politely, don't go "aha, I caught you in a lie!"
[Edited to add]
I've been rereading the older posts where people have been caught in lies and in the "Co-Irker Tried to Kill Me" thread EHellDame states "I believe it is legitimate to question someone about the inconsistencies of their story". Now she said that in 2008 so things might have changed since then and it is said in context to that thread.