It is actually quite possible...
I get a shock off of our 350L if I have my arm in the water and touch the light reflectors with my arm...
I only notice it if I touch it with a sensative bit (eg underside of my arm when I'm in upto my upper arm!)
also, if its just a one-time occurance, then it could just be static build-up. my tank gave me a bit of a scare yesterday when i got shocked but after trying (several times) to replicate it, i decided that it was just a static discharge