I've fixed a couple of networks that had split pairs. Barely works at 10mb, definitely not at 100mb.
The most common error that I see (when someone crimps their own 8p8c connectors):
1 white/orange
2 orange/white
3 white/green
4 green/white WRONG
5 white/blue
6 blue/white WRONG
7 white/brown
8 brown/white
All I can guess is that they look at a patch cable and duplicate the wiring, but don't notice how the blue pair is in the center and the green pair around it.
