My MBP's HDD is half OSX and the other half is Bootcamp.
While booted into OSX I decided to let Software Update install Yosemite. I left the room. Upon returning a while later I found the following error message: "a mismatch between the MBR and GPT partition maps is not supported".
I found an excellent post in this forum by @klanomath that dealt with a MBP stuck in an endless upgrade attempt loop like I'm experiencing, but that situation required reinstalling OSX; I'm hoping this "mismatch" can be repaired so the installation can continue.
If not, is it possible to interrupt the endless installation attempts and see if the existing OSX is still bootable? (has the existing OSX installation been destroyed by the upgrade yet?)
I found another post in this form, by @AndyD273, where it looks like a proposed solution exists but I do not know how to get from this endless loop state into the position where I can download, much less install, gdisk and use Terminal (and I cannot ask via comment in that post because I lack the "reputation"). All I know how to do is use the Option key to get to my Windows 8.1 partition.