I think the mistakes that can be classified as the 'biggest', are typically the ones made in personal relationships. Those are the things that probably cannot be mended/replaced with any other 'equivalents'.

In terms of career/work etc, i really do not think one needs to worry much or brood over past decisions, we all got plenty of time (hopefully) to work everything out as we want it to.

This quote sums up a lot of things nicely:

  The greatest mistake you can make in life is to be continually fearing you will make one. -- Elbert Hubbard