Sometimes you notice something really funny and you want to laugh, but it would be disrespectful; for example :

There's two escalators that go up out of Granville Station; they are located beside each other.
I was on the left one walking up and there was this girl on the other one.
While I was walking up, on the corner of my eye I saw her fall face first on the escalator.
She made a little "oomph" sound and I ignored it at first, but the more I thought of it the more I found it funny.
I began to smile but kept thinking to myself, "omg stop, don't smile, don't laugh".

It was hard. But I so wanted to laugh.