That an EO is excommunicated if they receive in a Catholic church. The same was Catholic practice before VII.
When my Ukrainian Greek Catholic grandmother passed away, my RO uncle & aunt did not participate in the Liturgy nor did they receive Communion.
When my RO uncle died, my parents told us we couldn’t participate in the service nor even say “Amen.” And as I’ve posted previously, my paternal RO aunt was a very devout RO. Seriously, she’d go ballistic if she saw a Catholic going up for communion in a RO church. (She died 1.5 years after my uncle.)
I could not in good conscience participate in an Orthodox service. It breaks my heart that we’re so close and yet so far apart.