Are there any undocumented bugs of the GB ?

Yes. There is the sprites bug that was fixed in the GBC and also the following that doesn't apply to the GBC.

In certain situations, writing to the STAT register ($ff41) seems to cause bit 1 of the IF register ($ff0f) to be set (and thus cause interrupt $48 to occur, if it is enabled). Due to programming bugs, at least two games (Roadrash, Legend of Zerd) insist on this quirk, and are incompatible with the GBC.

As far as has been figured out, the bug happens everytime ANYTHING (including 00) is written to the STAT register ($ff41) while the gameboy is either in HBLANK or VBLANK mode. It doesn't seem to happen when the gameboy is in OAM or VRAM mode, or when the display is disabled. (Info from Martin Korth.)