Even when error message was clearly understandable for my expertise, it took surprisingly long tome to switch from one mental activity - "Pay bills", to another - "Investigate technical problem". And you have to throw away all short memory to switch into another task. So all rumors about "stupid" users is direct consequence from how human mind works.
99% of the population have no idea what "Header size exceeded" means, so it absolutely is about understanding the message, if the devs expect people to read the error.
But I WOULD expect the user, when sending a message to support, to say they're getting a "Header size exceeded" error, rather than just say "an error".