Whenever I see questions such as this there is a high possibility that there is going to be two answers in one, and that creates a dichotomy of which the answerer could be right with one and wrong with the other. Although D&D 3.5 is the ancestor of Pathfinder, there are still different rules regardless of the crossover compatibility.
Unless a questioner is seeking both, should it be also incorporated into the question as such, without simply slapping on tags?
Tags should always be adjusted to suit the question actually asked. When someone tags with both and it's not 100% clear why from the text of the question, we'll comment to ask them to clarify what they were thinking when they added that tag.
In general, don't edit the question to reflect the tags until it's clarified whether they are actually part of the intended question. Often enough they're just guessing that the other game from what they're actually playing might be relevant — in those cases, they don't actually need both, and should leave it up to the answers to suggest that rules from the other game might be helpful.
In general, tags should reflect the game actually being played and the specific question being asked about it. If there are more game tags, the question should make it clear why and it should be a good reason. Without a good reason for using both tags, we remove the extra tags, as usual.
In the case of the question you're using as an example, that should have either been clarified by the asker in the first place, or had the extra pathfinder tag removed. Since it's an older question though, perfect tagging is not necessarily important enough to justify bumping the question back onto the front page, and I would normally wait for it to be bumped for some other reason (e.g., a new answer is posted) before making small edits like that.
Generally speaking, playing “3.PF” is extremely common, and initially, at least, Paizo touted “backwards compatibility” as a major feature of Pathfinder – that you could play 3.PF.
Now, the backwards compatibility isn’t as complete as it might be (they didn’t simply write new 3.5 feats/classes/spells/etc., they rewrote a number of the base rules and thus conversion is required), but it’s still nominally a feature, and in a lot of cases does more-or-less work out.
The issue ends up being that because the two are “close enough” for a lot of groups, they don’t explicitly define which way everything is going. Certain, “big” items, those get might get definition – “Pathfinder feat progression, but use the 3.5 version of feats found in both,” for example, is something I have seen at a number of tables.
But stuff that tables don’t pay much attention to until the question comes up? Things that we actually see questions about here? Those things are not defined ahead of time. Indeed, querents typically would not even know that the two differ on the asked-after mechanics.
It then becomes the answerer’s responsibility to address the conflict: to describe what each game states, with references as appropriate. It becomes the community’s responsibility to recognize the question’s requirements, and thus upvote only those answers that address both, and downvote those that address only one and ignore the contradiction between the two. The only good answers to such questions are the ones that get the details right for both. I strongly believe it is the community’s responsibility to downvote an answer that gets one right but the other wrong – with a clarifying comment pointing out the problem. It is also everyone’s responsibility to come back to the answer and un-downvote, and perhaps upvote as appropriate, after the answer is edited.
This makes such questions, when they touch on areas where 3.5 and Pathfinder have disagreements, somewhat more difficult to answer, but far from being too much to expect of this community.
