(conjecture and speculation follows which may or may not reflect reality)
I suspect 65535 (2^16-1) is the largest number the stack-count variables can handle cleanly.
The game usually puts a cap on the range of stacks at 60000 for nice readability. If something slips through (eg: the begging anti-gold bug mentioned by UOKaiser and Aurelius), the game can get confused in some calculations and flip over the top. Even the "1 gold coin" is the client going "wait, that number doesn't make sense, uh, let's call it '1'".
My opinion is that its best to dump them - they're more trouble as a something-broken that they're worth as a novelty.