The level limits on Demi-humans were a hold over from the earlier editions where they were more powerful than humans. You could argue that this difference had been lessened by 2e. However, as noted in the swords and wizardry rule book, the level limits are part of a package that genuinely distinguish between human and Demi-human characters.
148. The PCs are pestered by a madman who talks about stuff that isn't there, e.g. trees on the roofs of buildings etc. Later, they find an innocuous secret door in someone's house that leads to exactly the same room they just left. However, what they don't know is that they have entered a parallel but overlapping dimension, when they leave the house they can see all the stuff the madman sees. However, the differences all appear to be cosmetic.
If they remain in this dimension have things proceed normally until they are asked to meet a bearded man in a tavern. However, in their dimension he is clean shaven.
Bonus points if you can fix it so they end up with the party split across separate dimensions.
I read some many generic fantasy books now that I get more disappointed if I aren't dreadful.
However, there is one non-fantasy book which really disappointed me - The Catcher in the Rye - I'm not sure I can adequately express my hatred for the main character in this book without causing the swears filter to explode. On the other hand, it must be a pretty well written book to have caused such a reaction.