Recently, Super Junior's Heechul got distracted and forgot to get on a circulating platform with his fellow Super Junior members while performing at their concert in Beijing. (See DramaFever article with video HERE.) It's possible that he was simply tired. However, there is a popular theory circulating among Chinese SJ fans that points to what Heechul was looking at as the reason why he "zoned out." 

See Shin Ha Kyun as an elderly chaebol who ages in reverse in the romantic comedy Mr. Baek:

Chinese SJ fans have been referring to this photo to show that Heechul became lost in thoughts when he saw something in the audience. Where Heechul was standing made him the only one among SJ to see the lighted signs in the audience in that direction. The signs lit up Hangeng and Heechul's first names in Chinese characters, "Geng" and "Hee." 

Hangeng was one of SJ's original 12 members when SJ debuted in 2005. Heechul and Hangeng were the best buddies in SJ, but Hangeng left in 2009 after filing a lawsuit against SM and returned to China. Heechul admitted that he felt depressed at the time until his fellow SJs approached him and helped him out of the slump. In the years since Hangeng left, Heechul and Hangeng haven't forgotten their friendship, but they've hardly had the opportunity to get together much.

Many SJ fans were touched by this theory and felt it was quite believable because of Heechul's expression at the time. Many of them commented on how much they felt for Heechul and said that he is someone who values friendship.