The boys were in Japan for their fan meeting back in 2017. During the game segment, the members were supposed to draw the grouping sticks where members with the same color will form one team. Sehun was showing the stick he drew to the camera when Baekhyun sneaked up on him from behind.
The older playfully slapped the stick onto Sehun's arm and the maknae was not prepared for it, let out a "Ah!" that could be heard mildly without the mic
Sehun's expression was priceless too. Watch the moment at the 1:22:43 mark in the video below!