This has now been resolved, and we'll be publishing a more detailed postmortem on Monday.
Posted Jan 02, 2016 - 11:11 UTC
Identified
We experienced an infrastructure issue affecting a certain number of our API clients, which caused historical requests for the 1st and 2nd of January 2016 to throw an error during delivery. Earlier dates and realtime/latest data were unaffected.
This has now been resolved, and we'll be publishing a more detailed postmortem on our Status Page on Monday.
Please accept our apologies for any inconvenience caused.