This is in one of the big 5 (not specifying which for some anonymity)

I apply for an internship.
I get an interview.
I pass the interview and get the internship.
I do great in the internship. Get an exceeds expectations.
I apply for conversion.
I ace the two interviews.
I am told that the hiring committee gave me a yes.
I enter host matching (ie to find a team to join).
...
And that's it. I never get matched (I only met 1 team that had UI focus and I had previously asked to not be put on a UI team so the TL rejected me). 1 year later I'm told sorry the offer is no longer valid.

The annoying bit is that I decided not to apply to grad school and refused all other offers under the assumption that it was a guaranteed spot.