Tricky-to-emulate games

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Some Game Boy games depend on hard-to-emulate or just obscure behavior that some popular (or formerly popular) emulators get wrong.

Alone in the Dark: The New Nightmare
Poster child for hi-color techniques of rewriting GBC background palettes between scanlines.
Altered Space
Requires STAT IRQ blocking (if more than one STAT condition (mode 0, 1, 2, LYC) is enabled, all enabled conditions have to be false for a cycle before another STAT IRQ can get through); otherwise graphics are corrupt.
Boxxle
Loop at $1FB1 uses "nitro copy", writing a value to VRAM and looping until it took.
Daedalian Opus (JP: Bouken Puzzle Road)
Hangs if vblank interrupt timing is not precise.
Daiku no Gen-san: Kachikachi no Tonkachi ga Kachi and Tokyo Disneyland: Fantasy Tour
These rely on open bus in the SRAM area. Daiku no Gen-san hangs in stage 1-4 while transporting a block on an elevator to reach a switch. (mGBA issue) One minigame in Tokyo Disneyland accidentally reads a pointer from disabled SRAM and writes there. It relies on the value being $0A0A (or at least somewhere else where writes are harmless), not $FFFF (which is IE).
Game de Hakken!! Tamagotchi - Osutchi to Mesutchi
Complex multi-chip mapper.
Koro Dice
Timer glitch, write in second half causes extra tick
Pinball Deluxe and Pinball Fantasies
Depends on STAT IRQ blocking and dispatch to the correct handler if the least significant bit of IF & IE changes during interrupt processing. Otherwise, a cascading failure occurs within the engine that corrupts the playfield display. (See "Holy Grail" Bugs in Emulation, Part 1 and "Holy Grail" Bugs Revisited by endrift and LIJI.)
Pocket Bomberman (J), Pokemon Card GB (J); Robopon
Obscure "HuC" mappers.
Prehistorik Man
Large scroll text in title screen depends on mid-scanline writes to background palette (BGP) and uses sprite evaluation delay to change the effective horizontal position of these writes. Also uses the envelopes' "zombie mode" to change channels' volume without needing to restart a note.
Road Rash (original version) and Xerd no Densetsu
These run only on a monochrome system (Game Boy, Super Game Boy, or Game Boy pocket), not a color system (Game Boy Color, Game Boy Advance, Game Boy Player), because they depend on writes to STAT briefly enabling IRQ for all modes.
The Smurfs (SGB version)
Relies on double reading of opcodes when halt while IME=0 and interrupt is pending. At $0E3A, 76 f0 8c is halt ldh a, [$ff8c] without the halt bug. With the halt bug, it is executed as halt ldh a, [$fff0] adc h, and the game relies on the adc h.
Super Mario Land 2: 6 Golden Coins and Wario Land: Super Mario Land 3
These games use SRAM at $A000-$BFFF contiguously with WRAM at $C000-$DFFF to hold a decompressed level map, and they put most of the game's variables in SRAM. If an emulator automatically saves SRAM to disk a certain time interval in seconds after it was last written, it is believed that these games will practically never automatically save.
Warriors of Might and Magic
Draws status bar with mid-screen SCX/SCY changes. It also sets WX=0, which on GBC causes the window not to be drawn, and fills the window with garbage.

Game bugs

Hardware quirks and program errors cause some released Game Boy games to look wrong or odd on authentic hardware. Ideally, games should look exactly as wrong in an emulator as they do on hardware. Refer to this list if you're developing an emulator and a particular game looks wrong so that you don't go "fixing" bugs while breaking your emulator.

A Boy and His Blob in The Rescue of Princess Blobette
Z-fighting between boy and blob. This is common in monochrome Game Boy games, where X position overrides OAM index as determiner of priority.
DuckTales
Triggers a new wave channel note in NR34 without first stopping playback in NR30. This can cause wave RAM bytes 4-7, 8-11, or 12-15 to be copied over bytes 0-3.

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