The first possibility that comes to mind:
If you check out your scope, you should find that the ground lead on each probe (the small short alligator clip) is connected to the groung/neutral that you plug into the wall. So, you must realize that whenever you hook this alligator clip to something, you are hooking it to the ground/neutral of your shop/house/lab/etc. If there is not complete isolation in the circuit that you are testing, this can lead to a short-to-ground.
Second possibility that comes to mind:
If you use both chanels of the scope, remember that BOTH of these ground leads are connected to the ground/neutral, hence they are connected together. If you hook them up to two diferent parts of the circuit that are at different potentials, you are creating a dead short. As a rule, only one of the probes that I have connected to my scope has the ground lead attatched. I take the ground lead off of the second channel probe and leave it off to make sure I don't inadvertantly hook them both up and short something out.
I hope this info helps you track down the short that caused the problem.
Keep us posted.