eBay – Understanding Red and Green Prices in Listings

That's not how it goes. Any user can post a higher bid than your current one, there's only a minimun (the bid increment). If your maximum bid is still higher than the new bid, ebay will add the bid increment to the last bid and count it as your new bid, so you're still going to be the highest bidder, as long as no-one surpasses your maximum bid.

Now to why you shouldn't enter 99,999$ as your maximum bid... If your maximum bid is ridiculously high, you'll be sure to win the item, but you may end up paying a lot more than the original value of the product.
In your example, as long as someone is interested enough in the T-shirt to bid up to $100 for it, if your maximum bet was 99,999$ you'd find yourself paying $100.25 for a T-shirt that in other circumstances you'd only have paid $15 for.

I ended up ignoring the warning about not being able to post my listing and clicking the "List it" button anyway, and it seems to have gone through. So maybe the warning was displayed by mistake, and everything is fine? If I discover otherwise, I'll follow up here....