I was able to configure more than 16 unique HSRP groups on Supervisor Engine 2 prior to Cisco IOS Software Release 12.1(8a)E5 (running on the MSFC2), and now I cannot. What has changed?

In releases earlier than Cisco IOS Software Release 12.1(8a)E5, the software allowed creation of more than 16 unique HSRP groups on Supervisor Engine 2-based systems. This is a software bug that has since been fixed. If you upgrade from a release earlier than Cisco IOS Software Release 12.1(8a)E5 to a later release, and you have configured more than 16 HSRP groups, plan for this software fix by following the 16 HSRP unique group ID limit. Cisco IOS Software Release 12.1(8a)EX fixes this issue for systems running Cisco IOS Software on Supervisor Engine 2. This limit does not apply to the Supervisor Engine 1-based or Supervisor Engine 720-based system.