But also - when you sign your tax return, you are certifying that those contributions were made in the 2014 tax year. If you did get audited, as long as those contributions were made, where is the issue? My contributions are directly deposited from my paycheck pre-tax, so it is not left to chance.
The law specifically allows you to make contributions in early 2015 that count for the 2014 tax year, so making contributions before April 15 is not an issue at all.
The issue is signing your name to a tax return stating you have made contributions when you have not actually made those contributions. The bottom of Form 1040 says "Under penalties of perjury, I declare that I have examined this return and accompanying schedules and statements, and to the best of my knowledge and belief, they are true, correct, and complete." I'm not a lawyer, but it seems that there's a reasonable argument to be made that you are committing perjury (a felony) when you sign a tax return claiming contributions you haven't made.
That said, it seems exceedingly unlikely that you would be prosecuted or penalized in any way for doing this,
as long as you do actually make the contributions by the deadline. If you can't make the contributions for whatever reason, you'll have to file an amended tax return, pay more tax, etc.
So I would ask: what's your big hurry? Why risk it? Why not just wait a month or two to file your tax return until you have actually made the contributions you plan to claim?