You should have still accepted the other one first then told them your circumstances changed if you got a better job.

I don't agree with people saying OP should have taken the job offer then quit later if you got the other offer because that looks very unprofessional on your part. What OP should have done is let the recruiters from the dream job that he has another offer on the table. Not only will they learn of your situation but it will make you look better in the regards that other people want you, too. I was in the same situation when I was interviewing for internships but when I told them about my other offer, they offered me a position two weeks before they finished interviewing all the other candidates.

Your first mistake was putting all eggs in one basket. You should have taken the job offer so you HAD a job. Now say the other place wanted you, then you could have easily left the other job to go to it. I get that some people don't enjoy doing that but it's a better option than having no job at all

That's The breaks. You lose out being fickle

If it was getting close to the expiration date on the offer letter....maybe you should have taken it.