There are many, according to Nokia. Some have already been identified by critics of the agreement, under which Nokia will phase out use of its Symbian operating system in favor of Microsoft's Windows Phone software. Publicly held companies routinely disclose all the possible risks of their businesses to shareholders.

The immaturity of that software, just released in phones in November, is one serious risk, Nokia said.

"The Windows Phone platform is a very recent, largely unproven addition to the market focused solely on high-end smartphones with currently very low adoption and consumer awareness relative to the Android and Apple platforms, and the proposed Microsoft partnership may not succeed in developing it into a sufficiently broad competitive smartphone platform," Nokia wrote in the filing.

While the companies have signed a "non-binding term sheet," they still have to negotiate the final contract, a process that might take longer than expected or might not happen at all, Nokia warned.

By choosing Windows Phone, Nokia may forgo more competitive alternatives that would let its phones reach greater and faster acceptance in the market, it said.