I've read that if you overpitch, the main thing that can happen is that new yeast cells don't form because there's enough old yeast cells around, and therefore a) you don't get the flavors that develop during the yeast growth stage (probably you don't care about this for us-05) and b) you won't get full attenuation due to the lack of fresh yeast.
Normally this can occur when re-using a yeast cake. But in your case, doing 1 gallon batches, I guess it is possible. Mr. Malty recommends only .2 packet for 1 gallon of 1.048
This is just from reading, I've never had this problem myself.