What happens if I’m assigned stock at expiration, and my account doesn’t have the funds necessary to satisfy the margin requirement?

Übersicht: 

If an expired USD option position results in an automatic exercise (the Options Clearing Corporation will automatically exercise any stock option which expired 0.01 or more in-the-money), and the resulting stock position causes a margin deficit in your account, the account would become subject to immediate liquidation.  Given that the OCC processes the exercise and assignment after the expiration Friday close, liquidations in USD equities usually occur shortly after the open of regular trading hours (09:30 EST) on Monday or the next trading day.  Please be aware that any positions could be liquidated as a result of the account being in margin violation—the liquidation is not confined to only the shares that resulted from the option position.  For example, if the account holds currency, futures, future options positions, or any non-USD positions, such products may begin trading prior to Monday morning and, as such, liquidation of any of these positions could occur in order to meet the margin deficit which resulted from an options exercise.

Background: 

Account holders should refer to the Characteristics and Risks of Standardized Options disclosure document which is provided by IBKR to every option eligible client at the point of application and which clearly spells out the risks of assignment.  This document is also available online at OCC's web site.