Conflicting error messages of "scheduled maintenance" or various other errors, for upwards of 18 hours.
Yahoo Mail has been down for many users since at least the evening of December 9. Conflicting error messages range from "scheduled maintenance", then just "maintenance", to variously numbered errors, and Server issues. Massive number of complaints on Answers.yahoo.com. Attempts to contact Yahoo customer service through help pages have failed, as have calls, which are simply disconnected after a long hold time. Yahoo finally sent a Tweet through twitter.com/YahooCare, acknowledging the problem, and "aiming" to fix the problem by 1:30pm Pacific Time, but several hours later many of us are still locked out of our email accounts.
Other Yahoo services also unreliable, including Yahoo Groups not fully loading, and Answers loading slowly. All those recent "improvements" that no one likes apparently broke Yahoo.