When I was visiting Portugal I stayed with two people, one from Portugal and the other from Spain. They had been living together for months before I joined them, and speaking to each other in their native languages.
I was a unilingual Anglophone, and to accommodate me, they both spoke English in my presence. It was only through the medium of English that they discovered that they had misunderstood each other for months about various things.
The fact that Spanish and Portuguese were similar lulled them in to a false sense of comprehension, when in fact there are many false friends (cognates) and divergent nuances.
I'm told this happens a lot among Scandinavians too, and Danish is known to be especially difficult for Norwegians and Swedes to comprehend completely, so many fall back on English to avoid misunderstandings.