that their kid is ill before you see them...
Niece has a horrible cold and nasty cough (better not be COVID). We didn't know til we arrived to stay with the family (drove a few hours to get here and been stuck here 4 days). Now DS1 has caught it. He missed training in his chosen sport to come away for a few days, now he'll have to miss a week more to recover. If it's COVID he'll have to miss even more. He has a big competition coming up so needed the training. I'd never have come away if I knew niece was so ill.
Why do people do this? Drives me insane. When the shoe is on the other foot I always let people know 'little X has a cold, do you still want to come?', 'y has a cough, not sure why, but just letting you know in case you'd rather not come', etc ...
DS2 had a friend over a few weeks ago - again he was sneezing all over both DC, streaming cold, temperature. Mum dropped and ran. Both DC caught it. Now DS1 has missed training from this annoying playdate kid, and again he'll be missing it from annoying niece.
Wish people would not do this. or at least let you know and give you the choice?