In Korea, many men call their friends' wives 'jesu-ssi'. While it means 'sister-in-law' translated literally, the word is to call women married to a person younger (or lower status such as a sunbae to a hoobae) than they are. Therefore, the honorific is wrong because friends are supposed to be on the same level.
On the 5th episode of 'Journey to the West', Kyuhyun called Ahn Jae Hyun's wife Goo Hye Sun. When she picked up, he said, "What should I call her? I heard it's not a good to say 'jesu-ssi'." The others suggested, "Then call her hyungsu-nim," which also means 'sister-in-law' but for those married to a person older. Kang Ho Dong protested this, saying, "You guys are friends (same age), so why would you say hyungsu-nim?", but Kyuhyun kept his stance, saying, "I heard it was wrong to say jesu-ssi."
After the episode aired, many netizens praised Kyuhyun for keeping the proper etiquette. Others went on to point out that while the two are considered the 'same age', Kyuhyun is actually born in early 1988 and Ahn Jae Hyun is born in 1987, so Kang Ho Dong would be wrong anyway, and praised Kyuhyun for firmly keeping his stance even when he was prompted to do otherwise.