In the first section basically when the C4 is planted through the timber you cannot defuse it up close, you have to actually get a specific angle to defuse it when it is in this spot.
http://images.akamai.steamusercontent.com/ugc/647750614517830331/B140B9AD27C7183664DC7B5A9DED878C2D6FE123/
I can’t Url >_<