Like a Hair on ze Soup

"Tomber comme un cheveu sur la soupe."

Perfect French expression with no exact English equivalent.

The nearest is "fly in the ointment" but that is too strong & misses the "exquisitely bad timing" aspect.

Something which "falls like a hair on ze soup" is no great disaster, just a frankly unwelcome, incongruous, irritating detail, arriving at totally the wrong moment, which you can't just ignore, which you would unquestionably rather have done without, and which is going to spoil something you were all set to enjoy.

Like one kid coming out in a rash just as you have loaded the car to go on holiday.

Or your very friendly neighbour turning up with an offering of onions & the intention of a good natter, just as you were leaving for a great bike ride.

Or the phone ringing just as your favourite TV programme starts, although that is getting rarer these days.
Not because of the phone calls, but the TV programmes.

Parting thot: "One spot stains the whole dress." – Belgian proverb