Reneging on an Internal Job Offer

Here's the deal:

I applied for two internal positions after putting a year in a BO position at a BB. One of the positions an FO, and another an MO/ Product Control position. With the timelines for each being very different. I interviewed and got an offer for the MO position very quickly.

In the conversation, I had with the recruiter, I expressed my enthusiasm but NEVER stated that I had accepted. I assumed that only the signed offer letter would be construed as acceptance.

So I get my offer letter - and it clearly states that I had 5 business days to decide. Bizarrely, the next thing I know manager lets me know that I've accepted and I'm starting the new job in two weeks. Clearly a misunderstanding.

I call the recruiter and let her know what I was thinking and she was completely surprised and thought hat I had accepted and had been ready to go. Of course, it was just as much surprise to me. I then emailed my would-have-been boss and let him know the circumstances.

Now with all the above happening, I am worried that any shot at an FO offer is in jeopardy. I have had interviews, but no firm answer yet. Although I had never reneged (because I hadn't accepted in the first place) this might be seen as such from the recruiter's point of view. My question is, do the rules change for internal transfers across the board at all BB, or more generally, i.e, is the process more informal for acceptance?