PIC16F1776 interrupt triggered with no evident source
Are you using PIC's Interrupt-On-Change feature? There is a known bug in 18F HW. I'm not sure if it affects 16 series.

http://www.microchip.com/forums/m294169.aspx

http://www.xargs.com/pic/portb-change-bug.html


  So, we must read PORTB first, to clear the difference which got us here in the first place, and then clear RBIF, then read PORTB again (without clearing RBIF) to insure that any change on the pins between Q1 and Q3 that instruction is detected.