https://www.telegraph.co.uk/news/2023/12/06/mrna-jabs-modena-pfizer-quarter-unintended-response
mRNA vaccines (Covid vax) avoid being destroyed by the immune system on the way in (before making the target protein) by use of a non-natural amino acid substitution for one of the base pairs.
Turns out this causes a transcription error that, when it happens, makes a bogus protein instead of the target one. In the case of the Covid spike this bullshit protein is harmless (not sure how they know for sure) and is chewed up by the immune system. But in theory another vax working this way could have an operational, adverse protein emerge from the stutter.