The most impressive fail I have experienced is that I  had been susupended from school for two weeks. 

When I was high school student, I seldom submitted homeworks because of bother.

So, teachers always scolded me but I had never given up. 

One day, I made a serious mistake.

I asked my friend give me a homework he had submitted and was returned from teacher.

I changed the name on it and claim to a teacher that I had already handed in it.
I asked a teacher to check a stamp meaning teacher's confirmation on it.

The teacher said,"this stanp is not mine".

I had forgoten that my friend belongs to a different class from me.