As the classic Excel add-in is not the strategic direction for Oracle, there hasn't been much incentive over the years to improve it and it shows as the bugs have been accumulating. Here is one pretty egregious bug that is so bad, I find it shocking that customers continue to put up with it. The bug occurs when the Essbase add-in gets confused when a user has two instances of Excel open (like no accountant would ever do that, right?) It is very easy to replicate as shown in this brief video:
By the way, the airplane on the screen is not mine (but my Cessna 210 is one year newer than the one in the picture!)
For what it is worth, Dodeca does not suffer from the same issue (nor any of the dozens of other Excel add-in issues Essbase users often see).