... What immediately comes to mind is the rain of toads in Summer Knight i.e. major magical imbalance ...
But those were REAL toads, not nevernever summonings/constructs. That's part of what signaled that is was a major issue.
... Curiously, he sneezed as it happened. I have no idea what to make of the sneeze. And the sneeze came first. Before he thinks of the pot ...
He sneezed, and felt achy, but nothing happenend.
Then Mister cuddled him, purring "like a pot of boiling water," and
then Harry sneezed again; this time, both achy AND a wave of exhaustion, and the pot arrived.
... Did Harry summon the pot of boiling water? That kind of makes sense. He was thinking about it. After/as it showed up a wave of exhaustion swept him as we've seen a few times when he's done a big spell ...
I think this is it, honestly; now the question is how/why...
I will go ahead and WAG that Harry has caught a wizardly variant of a "cold." Recall the never-never mites on Kirby & Andi, in
Day Off... based on that short, and this event, my WAG is that there are a whole panoply of nevernever/magical pests/vermin/parasites/etc which are mostly just minor annoyances, like colds and allergies and mites and etc... With the occasional magico-Ebola, etc.
Harry has just never heard of this stuff previously, because Justin felt no need to teach it, and Eb had other issues to address than delving into whether Harry had the fully-approved White Council curriculum...