It's happened to me twice.
The first time happened when I was working at a donut shop. We were closed Christmas Day and reopened on Boxing Day. I worked overnights, and the co-worker in question, we'll call her Jessica, worked early mornings. On the December 27th, when Jessica came into work, she told me how before the store opened on Boxing Day, another employee pinned her to the ground and tickled her mercilessly.
The second time happened a few years later while I was working at The Home Depot. I was in the breakroom on my lunch break when two employees walked into the breakroom, we'll call them Anna and Beth Anna was teasing Beth about something, I don't remember what. Beth turned to Anna and said "just remember, I know that you're ticklish now." Unfortunately, I turned to late to see the reaction on Beth's face.