alt Hacker News> Now an international team, led by Flinders University, have found that in a small number of people, the immune system can accidentally confuse a normal adenovirus protein with a human blood protein termed platelet factor 4 (or PF4).
Seems to have been a legitimate, very rare, side effect
https://www.flinders.edu.au/research/articles/covid-vaccine-...
It's worth clarifying that the adenovirus-based (viral vector) vaccines that article is discussing were a completely different technology from the mRNA vaccines.