stupidest thing ever done

So my best friend ask for me to throw her napkin in the trash bin but we're not allowed to stand while our teacher is teaching, and the trash bin is right beside her and I accidentally threw it on her back, My best friend and I looked at each other and I really don't know what to do that time my best friend laugh really hard (I didn't know she was trying not to laugh) so I laugh with her and because of that we got detention after school