Guild Wars 2

Other items in the Gem Store that have limits on how many you are allowed to use will have a “You can buy X more at this price” notice or a “You have already purchased the maximum number of this item” notice. But with Bank Expansions, you can buy these even if you have your bank maxed out.

Sure, you should check first, but if you see they’re on sale, nothing prevents you from making an impulse buy of an item you consequently find you can’t use.

I agree there should be a safety net as you describe, though there is a way to stay safe: go to your bank and click on the padlocked tab to buy. No more padlocks? You’re maxed. Same with bag tabs on alts.