Intramuscular injections can accidentally hit a vein, causing injection into the bloodstream. This could explain rare adverse reactions to Covid-19 vaccine. Study shows solid link between intravenous mRNA vaccine and myocarditis (in mice). Needle aspiration is one way to avoid this from happening.
https://www.reddit.com/r/science/comments/q1uud9/intramuscular_injections_can_accidentally_hit_a/
https://pubmed.ncbi.nlm.nih.gov/34406358/