I knew it
It's something so mundane yet a coincidence that no one would mind it (from the characters's POV). Bravo Gosho.
Wonder who will connect the dots first regarding the de-aging/shrinking effect? Haibara herself? Shinichi? Sera or her mom? Rum or Vermouth?
The mention of Haibara having a cold definitely feels like a Chekhov's gun. That said, I looked back at the chapters where Shinichi and Mary shrunk, and I don't think they had colds. If they did, it was subtle enough they didn't show symptoms.
Funnily enough, I could've sworn cold medicine was somehow involved in the antidote Haibara developed. Looking it up though, I think I just got confused by how the first time he took it, there was a mixup thinking it was cough medicine. (Also found that it was revealed her dad originally worked making medicine for a defunct company, Shirahato/White Dove Pharmaceuticals, via Professor Agasa finding an old package of cold medicine when Haibara was coughing. Could be a coincidental choice since it's such a common type of medicine, but still a neat connection.)