I think it's only fitting that the first post here should be this:
http://www.oldmanmurray.com/features/77.html

This is a bit long, but just think of doing the following in a Monkey Island or Leisure Suit Larry style adventure game.

"Here is the solution to Gabriel Knight 3's first major puzzle, in which you must rent a motorcycle.

Gabriel must disguise himself to fool the moped clerk.

You must combine several items to construct an adequate disguise and gain access to the motorbike. First, return to the museum and swipe the red cap from the lost-and-found box. , but Gabriel knows he needs it now and has little trouble stealing the hat from the box. With the red hat in hand, head to the church.

Look at the Abbe's house and notice him watering his plants with a spray bottle. Wait for the Abbe to move back into his house and grab the spray bottle...   When you emerge on the new street, you'll spot a black cat in the corner. Move Gabriel up to the cat and use the verb menu to examine and pet the cat.

Walk back from the shed and notice the cat is now on a ledge. You can attempt to pet or grab the cat, but Gabriel can't because the feline is just too high. Here's where the spray bottle comes in. Select your inventory and pick up the spray bottle. Use the spray bottle on the cat, and he'll leap down and run, again, through the small opening into the shed. When he runs through the hole, he left some hair on the piece of masking tape you placed on the hole. Pick up the masking tape, and you'll gain black fur in your inventory.

Return to the hotel now and collect any items you missed the first time around that are vital to the disguise. These include the black marker from the hotel desk (just make sure Jean is wandering around), a piece of candy from the table near the lounge,

Head upstairs and knock on Mosely's door (room 33). He'll let you inside. If you want a hint about what to do with the candy, you can offer Mosely the candy, which he'll gladly take and consume quickly. Also, talk with Mosely about his passport, the key to solving the disguise puzzle. If you give Mosely the piece of candy, you must return downstairs and grab another one.

Locate the painting over the table depicting the street scene. Use the piece of candy from your inventory and place it on the table. Head down either staircase into the lobby. Look to the left of Jean's front desk and spot the room buzzers. Examine the buzzers and press the one for room 33, Mosely's room. This will buzz Mosely down to the front desk, but he'll become sidetracked by that yummy piece of candy you left for him.

Ascend the stairs on the right side, so you're on the opposite side of Gabriel's room entrance. Follow the camera around to Mosely's room and watch him exit and walk to the table with the piece of candy. Mosely will bend over and grab the candy, gobbling it up like before. Walk Gabriel over just behind Mosely and use the mouse cursor on Mosely or his passport to pickpocket him and swipe the passport.

As soon as you've got the passport, quickly head to Mosely's room 33 and enter it. Nab his gold coat on the coat rack by using the verb menu while the mouse is over the coat. Place the coat in your inventory and exit Mosely's room.

This sequence could take a few tries to get everything right, but you can repeat the process as many times as necessary to secure the necessary items: the passport and gold coat. Just use more candy and keep pressing that buzzer.

Open your inventory now; make sure you have the black marker and syrup. Grab the black marker and use it on Mosely's passport to make a mustache. Next, grab the black fur from the cat and use it on the syrup to make a black mustache. Finally, use the red hat on the mustache and then on the gold coat to complete your Mosely disguise.

With your disguise ready, return to the moped rental shop."

Gee, if they're going to make it that obvious, it's a wonder how they ever sold any walk-though guides for the game.

Thought I'd bump this once and see if anyone here actually care to take the time to read it.

I read it.  I've never had major problems with this puzzle, but maybe that's just because I managed to figure it out on my own or that I have been playing these types of games for so long that I have just come to terms in accepting crazy adventure game logic.  I am a fan of the adventure games, but I feel that this kind of thing is just the nature of the genre.  Puzzles in adventure games are always about trying to figure what the developer wants you to do.  You are presented with an obstacle that would have multiple solutions in reality, but in the game you must use the one solution created by the designers.  I play adventure games for the stories and to hear a story like the one told in Gabriel Knight 3 is well worth having to deal with crazy puzzle logic every once in a while.