At my old work place there had to be a certain number of staff in each day and you weren't allowed to book time off if it meant the number went below this level.
One of my colleagues was a nasty witch, always trying to get people into trouble, was lazy, two faced etc and had a particular problem with me so I absolutely hated her.
One beautiful sunny morning her and another colleague came into the staff room, they didn't notice me and continued their conversation with the witch saying "It's such a lovely day, I can't be bothered with work, I've checked the rota and there's enough people in so I'm going to book the afternoon off"
I quietly snuck away, told the manager I wasn't feeling great and asked for a half day which was approved.
The witch then waltzed in saying loudly "I'm booking a half day today seeing as there's plenty of us in and then I'm going to spend the afternoon at the beach " with a smug smile.
Her face was an absolute picture when she was refused her afternoon off and she was even more livid when she discovered that it was because of me being "poorly"
Even funnier the next morning another colleague asked in front of the witch if I was feeling better and I replied "oh yes, in fact I felt better soon after I got home so decided to spend the afternoon at the beach, it was fantastic!"
So petty but well worth it!