The program that the game's artists used to create characters was called Deluxe Paint, and it saved files as "brush" files. When drawing the main character, he was saved as guy.brush, hence the name Guybrush. His last name, Threepwood, was decided by a company contest.
If Guybrush walks to one corner of the lookout plateau on Monkey Island, the rock will give way under him and he will fall, supposedly to his death, and a menu screen with the options "Restore", "Restart", and "Quit" will appear briefly. This is a parody of many Sierra games such as King's Quest IV: The Perils of Rosella (1988), which feature many inconvenient ways to die, a thing that had always frustrated designer Ron Gilbert.
The man inside the troll suit guarding the bridge on Melée Island is a pixelated rendering of George Lucas. (This is more obvious in the enhanced graphics of the HD remake The Secret of Monkey Island: Special Edition (2009)).
The troll guarding the bridge asks for something "that will attract attention, but have no real importance" as payment to cross the bridge. The item he accepts is a red herring. In storytelling, a 'red herring' is a plot device often used for misdirecting the main characters and the audience; it is usually a person or item that is set up to attract attention, but ultimately turns out to be innocent or of no importance. A red herring should not be confused with a "McGuffin", a term invented by legendary suspense thriller director Alfred Hitchcock for an item that is the focus of the plot, but its real purpose or nature is never revealed (the Treasure of Big Whoop from Monkey Island 2: LeChuck's Revenge (1991) would qualify for that description).
In the Swordmaster's forest, there is an area with a tree stump. When you check it out, Guybrush mentions that there is a hole in it which leads to a maze of caverns. When you try to go into the hole, the game will ask the player to subsequently "insert disc #23", "#47" or "#114". Afterwards, Guybrush says he'll skip that part of the game. As the game was originally divided over only eight floppy discs, this was meant as a joke. However, many gamers did not get it, also because there was a special credit at the end for the "art and animation for disc #23". LucasArts' help desk received so many calls about the "'missing disc," that the joke was removed from the CD-ROM version of the game; when the player tries to climb down the stump, Guybrush says he won't fit through the hole. References to this infamous joke were made in Monkey Island 2: LeChuck's Revenge (1991) and The Curse of Monkey Island (1997) (see the Trivia pages of those respective games).