First, here's how I understand this sentence: Have you ever thought that this is common sense, or been said by other people that this is strange, or thought that this is strange (or marvelous?) What situations are/were these?
- I guess this で is a connective form of です?
- I think こと is in a pronominal function here and represents what was described before.
I can't remember having encounter these two points before. I also feel unsure about my translation, because I've never used "or" instead of "and" or similar for this kind of connection. But with "and", while it wouldn't be completely wrong, it feels slightly off.