Let me explain what I mean: Yesterday I wrote 2013 on a bank slip. Future historians may mistakenly determine that the butterfly wing-flap that started the downfall of American civilization happened in 2013, not 2014, and all because of this simple screw-up.
Historians, have you ever come across a document that you doubt has the correct year listed because of this brain fart that many of us occasionally commit toward the beginning of a new year?
I'll see this typo now and again in the archives, and it's never caused any doubt for me or a researcher that I can remember. Often people contemporary to the document (most likely secretaries) will make a little hand correction too. One thing to keep in mind is that historical documents very rarely exist in isolation -- if a series of letters goes Dec 1914, Jan 1914, Feb 1915, you'll figure it out pretty quickly. In the wild days before archival science it was also customary to bind important letters into volumes to preserve them, and in that case someone closer to the actual events would put them in chronological order. If it's a wild lone letter you can also date it from the contents of the letter, if they mention anything with a known time, or if the envelope is still around, postmarks never make this typo.
Sorry the answer is so boring!