R-M-W's themselves only have a problem when a pin in Analog mode is set as an output.

The real R-M-W problem comes when you do more than 1 R-M-W too fast, and there's capacitance on the Pin.

For instance, if you were doing the previous example but had 2 nibbles being put to the same port, 1 to the lownib and 2 to the highnib.
Code:
PORTB = (PORTB & %11110000) | (Nibble1 & %00001111)
PORTB = (PORTB & %00001111) | (Nibble2 & %11110000)
The last thing the first line does is write the result to the port. And the first thing the next line does, is read the port.

If there's any capacitance on those pin's, it's Very likely to cause the infamous R-M-W hiccup.
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