Bawaybaway,
I am sorry that that happened to you. It's terrible when it happens, but I don't think it's common. It is always a good idea to keep track of your own credits and check how the academic advisers apply things. I do not believe that they review past decisions when adding new credits, so a mistake by one adviser will not be picked up until final graduation review if you do not catch it yourself. I have always checked over all additions to my transcripts and corrected a few minor errors over the time I have been at Excelsior, from initial miscalculation of quarter credits to mislabeling a lower level course as upper level that came from a school with a strange numbering system. I really like Excelsior, but it is not a good idea to assume that mistakes won't be made in the additions to one's status report. I regard this kind of degree as involving a great deal of "do it yourself!"
Then again, many kids in traditional B&M's who are not diligent about reading the catalog themselves are surprised to graduate late because they are missing some requirement or other that "noone told" them about, so perhaps that isn't unique to this process.