Wonder Boy/YMMV

The series in general

 * More Popular Spinoff: The Monster World sub-series. There's a reason why the PS2 compilation was titled the Monster World: Complete Collection and not the "Wonder Boy Collection"
 * Title Confusion: There are two games titled "Wonder Boy III" and both are justified since they're the third Wonder Boy games for their corresponding platforms. Monster Lair was the third arcade game and The Dragon's Trap was the third Master System game. Officially speaking, the latter is known as Monster World II in Japan, where it is recognized as the fourth Wonder Boy game, since the following game (Wonder Boy in Monster World) was called Wonder Boy V: Monster World III over there.

Wonder Boy

 * Demonic Spiders: The frogs (which are extremely hard to hit before they jump into you) and the literal spiders that appear in droves in later stages.
 * Goddamned Bats: Bees, rocks, bats, octopuses, swordfishes, etc.

Wonder Boy in Monster Land

 * Porting Disaster: The Amiga version had stumpy graphics and tinny sound, did not label shop doors, and had a small play area to fit the status border. (The Master System version instead compressed the HUD into a small bar and put the rest of the info in a pause menu, resulting in a much more playable screen size.)

Wonder Boy III: Monster Lair

 * Porting Disaster: The Mega Drive version had its graphics horribly mutilated, even worse music than the arcade version, and five stages were removed due to lack of ROM space.

Wonder Boy III: The Dragon's Trap

 * Ear Worm: The dragon bosses' battle song. Enjoy: http://www.youtube.com/watch?v=eHeOUvA5CLo
 * This installment is well known among the fan community as having some of the most memorable music in the series.