PIC16 I2C interrupt does not occur if serial port enabled
This is not a silicon bug. This is me being a complete idiot.

What is happening is that my uart code was written a year ago and relied on the fact that PIR1, TXIF was high if the UART transmit buffer was empty. My code shovels data pointed to by FSR0 until it hits a "\0" and then disables TXIE. Thus when I load FSR and set TXIE, the data is automatically transmitted A year passes and I write an ISR like this:

    .intr   CODE        4

    ; Enhanced Midrange CPU does a context save on:
    ; W, STATUS, BSR, FSR, PCLATH
    ; retfie restores context

    pagesel     $

    ifbset      INTCON, TMR0IF          ; Timer0
      goto      Timer0_Entry

    banksel     PIR1                    ; Bank #0
    ifbset      PIR1, TXIF              ; Serial transmit
      goto      SerialTX_Entry

    ifbset      PIR1, SSP1IF
      goto      I2C_Entry               ; I2C Peripheral

    retfie                              ; Nothing else


Well that's total and utter nonsense. If any interrupt is received other than a timer one - so either UART or I2C, the SerialTX_Entry will be called. TXIF is always set if the UART is enabled and no character is beng held for transmission. So in the above example, I2C_Entry will never be reached.