The risk to an investor (option writer) that the stock price will exactly equal the strike price of a written option at expiration; i.e., that option will be exactly at the money. The investor will not know how many of his/her written (short) options he/she will be assigned. The risk is that on the following Monday he/she might have an unexpected long (in the case of a written put) or short (in the case of a written call) stock position, and thus be subject to the risk of an adverse price move.
See Also:
Risk: The potential financial loss inherent in the investment.