Can closing get delayed at the last minute over a missing signature?

Yes, this happens more often than people expect. A required signature — a spousal consent, a specific corporate officer, a landlord's consent to a lease assignment — is frequently built into the closing conditions or the list of required deliverables, and lawyers generally won't release funds or documents until every required signature is actually in hand. One missing signature can hold up an otherwise fully ready closing.

In practice, this rarely kills a deal outright. Lawyers on both sides typically deal with it through a short undertaking (a promise to deliver the missing signature within a defined, short window) or by agreeing to push the closing time or date back by hours or days through a brief amending letter. Whether that's workable depends on how essential the missing signature actually is and whether the other side is willing to proceed on that basis. If you're told closing is delayed for this reason, ask your lawyer specifically what's missing and what the plan is to resolve it, rather than assuming the whole deal is now at risk.