Red-black tree. Adding a red child to a red node with a black sibling
Assume we have a black node $N$ with black child $L$ and red child $R$. Thus we have a red and black sibling. Recall that in a red-black tree all paths from root to leaf must have the same number of black nodes. The path $N-L$ contains two black nodes, the path $N-R$ just one. That means that both children of $R$ must be present, and must be black. So indeed, we can not add a new red child to $R$ because it already has black children. However, what can happen is that one of these black children turns red as consequence of a flag-flip. We then are in the situation you describe.