An error response contains a different HAPI version than other responses. For example,
curl -s "https://supermag.jhuapl.edu/hapi/about" | jq '.HAPI'
"3.1"
curl -s "https://supermag.jhuapl.edu/hapi/capabilities" | jq '.HAPI'
"3.3"
I think they should be the same version.