In Medieval era, used military ability to create an army of 5 Peasants with 28 CS, all of which were flagged at upgradeable. But...each could be upgraded to a Spearman with only 19 CS. Of course, this is in fact a downgrade that costs 108 gold.
Step by step of the bug
Step 1
Enter Medieval era, choosing Norseman.
Step 2
Use the military ability to create an army.
Step 3
Select one of the peasants in your new army, and hover over the upgrade button.
Step 4
Notice whether the "upgrade" is truly an upgrade...or a downgrade; maybe to Spearman.
In Medieval era, used military ability to create an army of 5 Peasants with 28 CS, all of which were flagged at upgradeable. But...each could be upgraded to a Spearman with only 19 CS. Of course, this is in fact a downgrade that costs 108 gold.