I don't recall exactly. I'd been away from much use of Excel for a few years, then had occasion to use it helping someone else. Whereupon I suddenly went from an early 200x version (lastly 2003, I think?) to 2007.

The ribbon was annoying enough. But then I started manipulating using shortcut keys. And a subset didn't product the expected results.

I spent some years working heavily in Excel, including extensive on-the-fly worksheet structure modification (shoving data around, adding and deleting rows and columns, copying values or just the "text" portion of cells, etc., etc.). So those shortcuts were second-nature, or "muscle memory", as they say, if a bit stale.

It was a one-off, helping that person. I currently do a little budgeting in Excel, but without much such manipulation, so I haven't continued to trip across and so recall in detail what broke.

I do continue, in my more spiteful moments, to hope they reserve a special level of hell for the people who make such UI changes. For a subset of people, Excel is a heavy-duty industrial tool, and dicking with the UI is tantamount to dicking with their livelihood, not to mention peace of mind. At a minimum, implement and insist on backward compatibility (as an optional mode, at least), no matter how much your "rock star" designers piss and moan, or your Management pisses and moans about supporting multiple interfaces. You have a responsibility to the user base that has propelled your product to such success while integrating its functionality into their subconscious actions.