If you accept and don't follow through your acceptance, you burn your bridge with that prospective employer - There is no other cost to you that I see.
It's really up to them whether they have a "do not hire" list of those who jerked them around and they put you on it, or whether they'll let bygones be bygones within 6 months to a year.
I lean toward them letting bygones be bygones because in business as in politics, holding a grudge forever is silly, unless the offense given was unusually serious and taken personally. If you take the offer and tell them you won't show up, they'll turn around and make the offer to the No. 2 prospect, so it's not as if they have to do their candidate search all over again. On the other hand, if they made the tactical mistake of sending letters of rejection to everyone including the No. 2 prospect immediately after you initially took the offer, they will most likely be seriously unhappy with you.
I'd say, stick with the first offer you accept unless the second offer is unusually good in some way for you and you can legitimately defend taking the second offer and nullifying your acceptance of the first offer - "it would cut my commute time from 3 hours to 1 hour" or "I like your company but the second offer is from Google".