Protocols like SPI, I2C, and UART are intended for chips sharing a PCB, lots of problems crop up when you try to send them over long distances. Michu designed individually controllable LED modules controlled by SPI signals sent over long lengths of cable. Initially he had lots of noise and buggy control. After some debugging and rewiring he got the system to work by eliminating ground voltage differences and buffering the signal. The link has a great writeup of the problem diagnosis and various solutions.
Via the forum.