My girlfriend, a PhD student, has been looking for an internship for this summer, mostly in government or at national labs. A week ago she got an internship offer from a federal agency, and they only gave her 24 hours to decide. But she had a positive experience interviewing with a national lab she really wanted to intern at, so within that 24 hour period, she emailed the national lab asking for updates, hoping to get more info. She didn't get a response until a couple of days later, by which time she accepted the offer from the federal agency.
Now today the national lab sends her an internship offer. She is debating whether to renege on the offer she accepted a week ago, especially since she has not yet signed any sort of contract with the federal agency. It is unlikely that she would want to work at that specific agency in the future, so I don't think she's concerned with burning a bridge with that agency. However I don't know how much information one government institution shares with others, so I don't know whether it would ruin all her prospects searching for full-time government jobs next year. (the federal agency that extended an offer is a minor one related to banking, not a major three-letter agency like the FBI or NSA). What are potential consequences to consider if she reneges? How damaging could it be to her future career if she does this?