Running a playtest game for a friend on rpol, and the system went through some big changes, including a shift in core mechanics - from a white wolf style attribute scale to a D&D type setup. Discussing how to do stuff with my player gave a resulting conversation which looked something like this:

Player: "How do I do initiative? Dice equal to my Dex right?
Me: "Well, its changed it bit. Just d20 + Dex modifier now, sorry."
Player: "OK, what's the modifier for a Dex of 2?
Me: "2 ?? Your elf is a gelatinous cube now??"
Player: "OH! I see. I may have to redo the Dex then."
Me: "Nah, looks fine to me..."
Player: "Well, do I get the engulf and dissolve abilities?"
Me: "Well, buying a 2 Dex should leave you lots of points, so you can probably afford to buy those."

This is obviously a particularly silly example of conversion error (I'm not actually going to force him to play a gelatinous cube ) but it did make me wonder what sort of blunders people have seen when converting between rulesets - either in their campaigns or just rulebook examples ?

In books, I remember being told about d20 Seventh Sea having a 55th level NPC, thanks to each of the several swordsmanship schools -which a character could actually learn a number of, with difficulty, in the original rules -becoming a separate prestige class.

Another case might be how half of Faerun's major NPCs seem to have gotten themselves xp penalties in 3.0, thanks to the changes in the multiclass rules (e.g. in the Epic Level Handbook Elminster -20%, Storm Silverhand -40%; in Forgotten Realms book Drizzt gets a -40% as well).

Alot of the blunders I have seen seem to be with D20, but I'm not sure if this just due to D20 rules being more specific in how they work, or from its tendency to get into everything like sand at the beach.