Suppose we have white T-shirts (index one) and black T-shirts (index two). So, we have one white T-shirt of size \t{S}, and also three white T-shirts of size \t{M}. There are three black T-shirts of size \t{S}, and also three black T-shirts of size \t{XL}.

Consider each winner:
\begin{enumerate}
\item The winner needs a white T-shirt of size \t{S}, we have one, so we give it to him. This was the last such T-shirt.
\item The winner needs a black T-shirt of size \t{XL}, we have one, so we give it to him. We still have two such T-shirts left.
\item The winner needs a white T-shirt of size \t{M}, we have one, so we give it to him. We still have two such T-shirts left.
\item The winner needs a black T-shirt of size \t{XL}, we have one, so we give it to him. We still have one such T-shirt left.
\item The winner needs a white T-shirt of size \t{M}, we have one, so we give it to him. We still have one such T-shirt left.
\item The winner needs a black T-shirt of size \t{M}, we don't have one. However, we have a white T-shirt of size \t{M}, so we give it to him. We no longer have any white T-shirts of size \t{M} left.
\item The winner needs a white T-shirt of size \t{M}, we don't have one. We also don't have a black T-shirt of size \t{M}, so we can't give the winner a T-shirt of size \t{M}. Therefore, we look at the next size --- \t{XL}. The winner wants to get a white T-shirt, but we also don't have such a T-shirt of size \t{XL}. So we give him a black T-shirt of size \t{XL}. This was the last such T-shirt.
\item The winner needs a black T-shirt of size \t{S}, we have one, so we give it to him. We still have one such T-shirt left.
\item The winner needs a white T-shirt of size \t{XL}, but we don't have either a white T-shirt of that size or a black one. Therefore, since we don't have T-shirts of a larger size, we can't give the participant the T-shirt he indicated, we record it on the list.
\end{enumerate}

We are left with two black T-shirts of size \t{S}.

We could not give out one white T-shirt of size \t{XL}.