The pic and most other cmos have intrinsic diodes in them the pull up
resistor gives forard bias to these and the chip works but some times in a
very strange manner. I have fallen into this hole more than once just when
you thaught it worked properly suddenly it didnt.

The only pin that hasnt got the diode is RA4 on '84 or equivelent pin on
other devices the one that doubles as Vpp

Cheers Steve.....