There is a general three year period of limitations, so for something minor like this, the IRS cannot go back past three years. The IRS has to request the prior years' information and/or audit you to go about getting that money.
Your defense for any year, past or present, is going to be your documentation (the tax code has documentation requirements in general). Without documenting and saving your general information for three years, you are typically leaving yourself exposed to an extent. Paying the current year amount is conservative compared to fighting it, and I have some doubt as to what the IRS would do, but whether you pay this year or not, each year (transaction even) is going to basically stand alone*.
*When settling with the IRS, documenting any year's acceptance or rejection of a treatment can have implications as to future/prior treatments, yes... If there are 10 things wrong, 'we' settle on 5, well then you better believe I continue right on with the other 5 things having passed audit the first time, for example... the battles you lose though, you do tend to 'lose' under audit going forward, but only "potentially" going backwards (The IRS agent can go back three years per above).
Going forward, I presume this will not be an issue because you will keep proper documentation. Let me know if there is something more I can clarify though.
Thank you for your question.