What we need to determine is whether ignorance of the law gives one a pass. It may be perfectly normal to an American to ask someone "What do you do?" at a cocktail party, but the other person may come from a culture where that is considered too personal a question upon first introduction.
I think it does, especially in situations that are not absolutely obvious.
I think everyone should check the appropriate attire when visiting another country (so they don't wear tiny shorts and a tube top in a Muslim country or to a Catholic church and so on), but it takes time and a lot of effort (not just a guide book) to figure out the finer points of etiquette and I think a different use of cutlery (switching vs. no switching), perhaps even using the wrong form of address, a couple of party throwing things I won't get into on Ehell (
) and so on should be excused - the offending party very likely didn't know they did anything out of the norm, if they are very new to the culture. If close to the offending party, it might be the kind thing to bring the difference to their attention (well, with the cutlery, I don't really think it matters, so I'd never mention it).
ETA: this is speaking as a person who has lived in two countries not her home country and has been repeatedly told that this, that or the other is done differently where I am that wher I grew up. I really appreciated it and I also appreciated the fact that people gave me the benefit of the doubt when I operated under my old rules...