Apologies for mangling any of the terms, I'm somewhat new to this.

Also, of course I'll take this up w/my LBS, but I'm the type who tries to figure things out on his own as well.

While riding my 2011 Felt Z6 after a recent general maintenance by my LBS I started hearing a noise.

I paid close attention and the noise only occurred when pedaling, not coasting.

I finished my ride and checked out the bike when I got home. That's when I noticed the crank arms slide back and forth, by almost 1/2 inch I'd say!

Question: what's going on here? That is, what's wrong such that the cranks slide back and forth like this? It seems clear the problem arose after the maintenance; is this a common error or situation?

Followup: could any damage have been caused? I want to know what additional questions to ask LBS beyond having them fix what I'm pretty sure broke only after their maintenance.

Thanks for any advice or feedback you may have.