Peter J, I can answer this because as an Orthodox Christian, I’ve done it…gone to a Catholic Church & received Communion - it is a sin. My spiritual father told me to never committ that serious sin again. Later on, I did it again, my spiritual father let me know that this is a very serious situation and basically that he is responsible for my soul/my sins and that by re-admitting to communion with the Orthodox Church he will be held responsible if I continue to do this & he continues to allow met go back & forth - he forgave me, but with strict warning.
(on a side note: as I’m typing this I’m realizing how the priests I confess to in the Catholic Church don’t seem to have nearly as much concern for my spiritual well-being or express their responsiblity for shepparding me & none of the Catholic priests have ever bothered to learn my name - I’ve never been forgiven by name in the Catholic Church as I was in the Orthodox Church)