Anatomy varies. In an accidental IV injection the dose will be dropped straight into the blood supply, and within minutes will be spread throughout the body, potentially ending up in tissues like the heart, ovaries, or pancreas, where it is not designed to go. If heart cells, for example, make the virus spike and display it on their surface, it's quite understandable if our immune systems think that look foreign, and generate a reaction. That's myopericarditis.
Not all veins are identical
Spread the message about "aspiration". This used to be standard practice for most IM injections but has been dropped over the last couple of decades.
To aspirate, a nurse would put the needle in, then briefly pull back on the syringe. If the end of the needle is in a blood vessel that brief suction will pull blood into the chamber. If that happens the whole syringe, needle, everything should be retracted and tossed in the bin.
Two studies now suggest that accidental IV injections may be responsible for increasing the risk both of myocarditis and blood clots.
A study in mice by Can Li showed that the mice injected "IM" did not develop myocarditis, only the mice injected intravenously did. The markers of cardiac inflammation were also raised.