I am following Theorem 2 from https://arxiv.org/pdf/1710.09437.pdf
I am referring to this example https://imgur.com/hwfYZor
Let a be the justified checkpoint with greatest height. Suppose that half of the validators have made votes s1 and s2, while the other half made votes s3 and s4. If the validators come to consensus that the chain with checkpoint d is indeed the canonical chain, I believe it would be impossible for them to finalize any checkpoint on that chain without validators violating slashing conditions, since the validators that voted for s2 would have to make surround votes. Hence, the casper protocol would get stuck.