Mass Effect 3 is the culmination of the series in which all your progress and personalization comes together. But one vital element of making the experience tailored, your custom Shepard face, isn't working for some players.
Several users are noting the problem on the BioWare forums (via Kotaku). The problem hasn't been formally diagnosed, but it seems to be hitting people who imported from ME1 to ME2 and now into the third game. It doesn't seem to be affecting all custom face users, though, so the exact circumstances are still a mystery.
An EA help file suggests remaking the face if you run into problems, while other forum-goers have come up with tricky solutions. Producer Michael Gamble has tweeted that he will pass on more info as he receives it.
Remaking your face works as a solution, but it isn't the most convenient. If you want to avoid that particular hassle, and you're running into the error, you may just need to watch for a patch. In the meantime, report the problem to EA as more data will help them pinpoint it.