I realize that there isn't much to go on yet. I'll update this report if I catch it happening. But I've noticed that occasionally, my Sim will use a toilet in one restroom, and will then go to a sink...
Ideas
This just started happening to me yesterday. He used the toilet on the first floor and now walks upstairs to the second floor to use the sink. I recently downloaded a new house off the gallery and moved them in but I kept all of my furnishings from the old house. I know it is the matching sink and toilet because this is from the sleek bathroom kit and I only have it in the guest bathroom in each house.
