Here's a big one: Andrew Johnson, who I believe is truly worse than James Buchanan (tied with Woodrow Wilson for 2nd worst POTUS) and America in general allowing the sacrifices of the Civil War and Reconstruction to be vain, and letting the postbellum New South turn into a dystopian nightmare for the descendants of slaves.
The sacrifices of the ACW were *not* in vain. The North fought it to preserve the Union, and last time I looked the Union *was* preserved.
As for Reconstruction, that was never going to get anywhere in the short term, Johnson or no Johnson. People in the North just weren't interested enough to put a sustained effort into it. However, by getting the 14th Amendment into the Constitution, it gave Washington the power to support Black rights if and when it came to see them as worth promoting. So in the longer term it succeeded.