A C128 Z80 Addressing Anomaly, as per Herne Data Systems.

IM2 expects the high byte of its jump vector in the I register and the low byte correlated with the interrupt source to be placed on the data bus. But because the C128's bus is designed around the 8502, that isn't supported and the low byte could end up being anything. So $FC00 to $FD00 are filled with $FD to force everything to go through the interrupt handling vector at $FDFD. On a more standard Z-80 system, the low byte would be an even number from 0 to 254, but on some systems can also be 255, necessitating a 257 byte vector table.