Hilchos Geirushin 11:23

Let’s say that a minor girl who is not able to give birth was married based on erroneous information; it was later found that she is prohibited to her husband so the court forced them to separate. In such a case, she need not wait to remarry. This is an extremely unlikely set of circumstances and the Sages didn’t institute this rule in cases that are uncommon.

Hilchos Geirushin 11:24

If a man betroths a woman within her 90-day waiting period, he is placed under a ban. If he betroths her and then goes into hiding, he is not placed under a ban. If they finalize the marriage within the 90 days, they are forced to separate until the waiting period has elapsed, after which they are allowed to live together normally.