As described in the title,
At the last stage of an interviewing process, a technical interview for say job A was conducted by the job A's team leader and a company's employee introduced as an expert in a specific technology called B.
At the end of the interview the news come that both jobs are being offered, both job A and the current expert's role say job B.
Left with the impression of this being a somewhat strange way to offer it, but can't exactly pinpoint what would be the red flag in this case.
It became especially annoying given that the candidate is already an expert in B but had made before more-or-less clear that the transition into the area called A was something he was looking forward to. Perhaps this subjectivity is the source of the strangeness, and if so it may be unjustified.