Was that the reason why you just created "Display Overbuy Rule to Game Players" , @LaFayette ?
Some maps enforce it, others refund, some carry over the units, others you lose the units. Knowing which is useful so you don't wrongly rely on the game engine to warn you about purchasing too many units to then lose or not place them.
In my experience, an overbuy is a game losing move. Losing units to not being able to place is essentially giving the opponent one or two SBR without the benefit of even an AA chance. The tactical disadvantage, usually counting on those units to be there, then to find out post-non-combat can easily mean you'll have fewer units than planned and a key position/battle would then fold as you'll be short.
Long answer, less long, users should be able to tell how the game will function without 'just knowing'. It's a UX design principle that you should be able to look at something and figure out how it'll work. Having some sort of indicator to tell you, whether text or otherwise (something), what will tell you would help jive with that. I've also had the rude surprise on maps to realize it did not warn/prevent overbuy and then overbought units. It's a particular problem say on WaW when there are many factories that are spread out. It's actually a bit time consuming there to every turn count up factory capacity, just so you can be sure to buy max, but not too much.