A 20% error rate on 35,000 files isn't entirely a user problem. Yes, the user ultimately has complete control and the issue could have been corrected if the user had carefully verified the data. In that sense, it's a user problem. However, if the tool is so counter-intuitive that roughly 20% of a large sample people make the same mistake, it's Excel's problem too.
I wonder if an aluminum extension ladder analogy is a first on
Consider an aluminum extension ladder...
Suppose the locking mechanism on the ladder worked properly when the user carefully verified that it was engaged. The user has complete control. If the mechanism was so counter-intuitive that 20% of the users ended up making the same error and falling off, it wouldn't be brushed off as a problem with stupid or careless users. There's no question whatsoever that the manufacturer would be held partially responsible. Hell, if they sold 35,000 ladders and found out that there had been 100 accidents because of confusion about the lock, they'd yank the product off the market immediately and probably face lawsuits.