LOL, evidently a Roman Catholic can, and does occasionally attend other churches.
So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it.
Pope Pius XI, 1928
I will give you that Vatican II and its popes have made Catholics think that this is AOK. They have even encouraged it.