Swedish vs Norwegian is mostly playful banter. Our lives, culture, values and language are all very similar, so being mistaken for the other is no big deal as it doesn't have any particularly negative connotations attached.

The real thing is Denmark vs Sweden.

It's like England/France, rather than Britain/Ireland.

Not hurtful at all, just banter. I live in England and you got a similar thing going with Wales or Scotland. All in good fun.

Some countries people will be very upset. I think these are the more sensitive ones:

Pakistan/India

Iran/its neighbors

South Korea/North Korea (you'd be surprised how often people think South Koreans might be from North Korea)

Mexico/its neighbors

By "sensitive" I don't mean that people will get very offended, just that they might be mildly annoyed. Norway/Sweden as well as Netherlands/Belgium is mostly a playful rivalry.

Norway may well be considered the upstart little brother, or hick relative of the Scandinavian nations.

This partially because it was a long time the subservient partner of Denmark-Norway, and then later passed to Sweden as war-reparations after Denmark backed Napoleon (never mind the earlier event where Trøndelag was given to Sweden after the 30 years war, only to be handed back a decade later because Sweden found them to be unruly).

That said, mostly it is playful banter. For example, all nations have similar jokes where representatives of the nations try to one up each other, and the nation of the story teller always come out on top to a massive degree.