Ok, so this situation is probably very rare, but I can see it potentially happening in a combat-heavy game.

Lets say a Protoss player Mind Control's another player's unit. Then, during combat, it's time to use that unit. However, lets say, for argument sake, that the other player has a HUGE hand of cards. There is one card left in the deck - and it's a REINFORCEMENT card. (Or there could be multiple cards that are all reinforcement cards, and maybe even reinforcement cards in the discard pile). How would this be resolved?

The same question could actually be asked without Brood Wars if a player chooses to draw from their deck, and all they have is Reinforcement cards left. However, that case is less likely since if all they have is reinforcement cards, that means that they have in their hand some useful cards for the combat.

As per "Singleton Reinforcement Cards" on page 30 of the SC rulebook, if a lone reinforcement card is played it is to be replaced with a newly drawn from that player's combat card deck.

As per "Running out of Combat Cards" on page 25 of the SC rulebook, as soon as the last card in a combat deck is drawn, the discards are immediately re-shuffled and form the new combat deck (although the rules don't explicitly state that this also applies if another player than the owner draws that last card, this has to happen as well - I think that shouldn't give rise to much discussion though).

Since every race's initial combat deck consists of 20 cards, out of which 2 (terrans and zerg) or 3 (protoss) are reinforcement cards, that leaves a minimum of 17 regular cards. Since player's maximum hand size is 6 (zerg, protoss) or 8 (terrans) and can't possibly grow to 17, there will always be some combat carsd that are not reinforcement cards in the discard pile, and consequently available for later drawing.

Edit: It is possible for the hand to grow that large (for terrans, it has to be 18) after all, if we're thinking Arcturus Mengsk and his star order, and that player having 8 cards in hand and then placing 4 research orders (gold research order plus star order as research order), not having him buy or have bought any tech that has combat values, and then letting him battle before the regrouping phase of that same round. By then, he'd have 20 combat cards in his hand, and with some bad luck could have no combat cards with combat values in the discard pile. I think that's fairly improbable, though

If there is really no other card - just draw one from his hand. You won't get any information you don't have yet (since all but one card are on his hand), and he can simply reshuffle his hand after that.

haslo said:

I think that's fairly improbable, though

Improbable, yes indeed. However, I have seen on multiple occasions a player (usually Terran, though sometimes Protoss) that has all but a few cards from their deck in their hand.

I'm inclined to go with Stefans answer if, in the bizarre chance, it happens Just pull one randomly from the hand.