It sounds like an interesting bug, can you elaborate?
On the surface it sounds like your new hire merely used fold expressions to call functions and operators that were already treacherous on their own.
Sorry, I can't look it up right now, but trying to reconstruct it in my head it must've been something like
y = (f(x1), f(x2), f(x3))
vs
y = f(x1, x2, x3)
Not entirely sure anymore though. But it was something about causing side effects but throwing away return values with the ,-operator and involved function calls, I think