Excel 2013 VBA Code not Backwards compatible with Excel 2010

I didn't have time to fully look at the problem, but it looked like it was mostly Compile Errors: Can't find Project or Library.

Also seemed to mostly be in my standalone userform calendar, which is opened by another userform. It works without issue on Excel 2013, so I was wondering if it was common for VBA codes to not be backwards compatible between the two.