Originally Posted by Fat Thorhammer
Again, the only way that can happen is if the Pokemon isn't neutral to Rock. Which matters, but it's possible to control.
Spikes and Toxic Spikes could be a problem if it's trying to copy something like Gengar, though.
Also abilities like Pressure and Mold Breaker that the disguise would normally have and the lack of its messages.
However, if the foe is distracted, he won't notice these changes