In terms of things that really happen:
Bring a grade school aged child who doesn't remember meeting the deceased, explain nothing about what the ceremony means, and let the child roam unsupervised.
Make a scene by sobbing complaints about your own unpopularity.
An aunt who had married into the family pulled both of these stunts at my grandfather's funeral. First the young cousin leaped out from behind a door and went "Boo!" at me in the funeral home. I took her aside and talked to her in a respectful way, explaining our Grandmother's loss, and the kid was an angel from that moment onward. She just needed perspective. Then when the kid's mother started sobbing I took the aunt aside to a back room and held her hand feigning sympathy through twenty minutes of self-pity.
Managed to keep that branch of the family from ruining the funeral. Yet I was only 22 years old and was quite close to both grandparents. Didn't get the chance to mourn him properly because I was too busy running interference for Grandma's sake.