As I was trying to max out my Roth IRA for 2014 at the Vanguard website today, it turned out I couldn't contribute as much as I thought I should be able to. Two contributions from early last year (totaling $800) which I had attributed in my mind to 2013 were categorized by Vanguard as 2014. Long story short, I learned you have to tell your investment company if you want to attribute a contribution to the previous tax year. Usually they ask when you're making a contribution between Jan 1 and April 15, but this was a recurring/automatic contribution.
So check your form 5498's, people! (They detail contributions for the previous tax year and usually come out in May.)
I know this is a N00B mistake, but I hope by sharing it here I might prevent other N00Bs from making the same mistake. Or at least the more seasoned investors can get a good chuckle.
The silver lining is that Roth IRA contributions are not reported on tax returns, so I don't have to go back and do an amendment or anything. (But save your Form 5498's, people!)