You should look into what the consequences are before you decide. As I understand it (though please take what I say with a grain of salt, as I may be wrong), it is an employment or contract issue. Any potential consequences would stem from that. Contrastingly, no consequences would come from the law society, for example (which would be more serious and scary, at least in my eyes).
I think that if someone accepts an offer, it creates a binding contract. But if you break it, it would be up to the employer to actively dedicate the time to come after you. We can think about the likelihood of that happening. I would tend to think it is unlikely. I know you are talking about summer employment and not articling, but for what it's worth, I have been told that people switching up their articles is something that happens all the time.
Reputation is the other consequence that a lot of people on this thread seem to be talking about. I honestly don't know how much we are supposed to factor reputation in our decisions before it stops being strategic. But surely there comes a point when it stops being smart to let reputation sway your decision. If I accepted an unpaid articling gig and then say, two weeks later, got an articling offer with a 7-sister on Bay St. / clerkship / insert-amazing-dream-job-here, I don't think I give enough craps about my reputation to stop me from taking the better position. How much better is your first choice from the position with the in-house counsels? If one is not all that much better than the other, then maybe reputation can play a bit more of a role.
The legal field is a dog eat dog world. Law firms mislead students about jobs all day every day and I don't see them losing sleep over their reputation. Look out for yourself and do whatever is most strategic.