I mailed a check as a donation to a charitable org on 31 Dec 2014. It was cashed on 9 Jan 2015. I intended it for 2014 taxes, but forgot to include. Since it was cashed in 2015, can I include it on my 2015 taxes? Does it depend on how the charitable org reported it?
When you write the check and mail it, the donation occurred from your point of view, no matter when it's actually cashed.
For example, from the NY Community trust's page on the matter:
A charitable gift by check is effective when the check is delivered or mailed, as long as the check clears in due course, even though a donor theoretically could stop payment at any time until the check is presented for payment.
So, if your check was mailed on 12/31/2014 it was in 2014's tax year. If you wrote it on 12/31 but didn't mail it until 1/2/2015, it was in 2015's tax year.
You can still file a 1040X to claim the deduction in 2014, if it's enough that you consider it worth your time to do that.