Well, it depends on whether they advise on the invitation that it's a dinner party, this or verbally let you know this either via email or phone.
If my friends (a couple) invited me to dinner, and I got there and realised there were more people there than I thought there would, being a bit of a social butterfly, I wouldn't really worry about it... would just think their notifying me must have slipped their mind, so I would make the best of a surprise situation and get on with the evening.
"Immediately my relaxed feeling
subsided and I went into professional mode destroying the whole relaxed part of the occasion. Do you know
what I mean ?"
Dave, I know what you mean , but depending on how well you knew the couple who invited you to dinner to begin with, and presuming you know them fairly well, I wouldn't be letting one minor misdemeanour ruin my outlook re- enjoying a night out with good company and friends thats for sure.
Perhaps it was a last minute thing, the couple in question being in town, this and were invited at the last minute and the host forgot to notify you?
In a friendly and informal 'relaxed' situation, I don't think this would bother me, I would be inclined to get over it and just enjoy the rest of the evening, surprise guests aside.
I like surprises though
However, if you are an image/socially conscious person, this and or are a little anxious about meeting new people, I can understand why this may have been a bit awkward for you.
Out of curiosity, how did your partner feel about the situation?