Just like you, I was looking at homes with my wife, in-laws, and kids. We were moving to a new city and my wife's parents wanted to see the area, so they came along with us.
In one of the houses we looked at, I decided it was time to take a load off. I excused myself to the bathroom while my wife and her parents talked to the realtor about the house. Nothing too eventful until I flushed the toilet, at which point it was immediately obvious that it wasn't going to flush. As the water climbed higher and higher, I started moving bath mats and anything else that shouldn't get wet. I grabbed bath towels and started making a dam to hold the water from spreading too far. Luckily, just water flowed over the top of the bowl, but all of my handiwork circled in the bowl like a great brown shark.
My wife knocked on the door to ask what was taking so long. At this point the cat was out of the bag and everyone had a good laugh at my expense. I had to ask them to find a plunger in another bathroom because there wasn't one in there. After finishing the cleanup, I left a few wet towels in the bathtub and asked the realtor to apologize on my behalf. I can only hope that she didn't divulge my name to the sellers.
We ended up buying a different house, but that site of infamy is on the way from our house to our kids' elementary school. My kids have given me a hard time more than once when we've driven past the scene of the crime.