In one case, I (the player) had an epiphany about the puzzle of symbology that was being constructed by the DM by the 5th adventure in a campaign just as we were breaking down the game for the evening. I did so so thoroughly that it would have revealed the campaign's overall metaplot...skipping ahead perhaps a year of gaming. Due to RW events, we couldn't game again for several weeks, and by that time, I forgot the structure of clues I had once assembled in my mind, and asked the DM to remind me.
He said no.
I said that, while it had been months of RW time- long enough for me (the player) to forget my insight- only 8 hours had passed in game, so I (the PC) wouldn't or shouldn't have forgotten what he figured out.
His response was that it was highly unlikely that my PC (whose mental stats were lowish) would have figured out the mystery in the first place, at least with the events up to that point.
He was right. While it was possible that he (the PC) could figure it out, I knew that part of why I solved the puzzle was because of my personal RW knowledge. I used knowledge and mental skills my PC simply didn't have.
So, to shorten an already long post, I'd have to say that even though I've been playing for 30+ years, I've always favored challenging the PC than the player. Going the other way has too many potential pitfalls.
Personally, I would call that whole long situation a pretty big "pitfall" to my gaming enjoyment. I would much rather just collapse PC/player knowledge and avoid the whole debate in the first place. It's precisely those complicated (and OOC) in-vs-out of game debates that are the single biggest drag on my playing a role and enjoying it.