Symptom:
Upgrades can fail on occasion with  /var/log/install/ora-install.log containing the following type of issue:

==== Log file content ====
Sat Oct 26 21:10:25 PDT 2013 :: DB state is

IBM Informix Dynamic Server Version 11.50.UC9X7 -- On-Line (CKPT REQ) -- Up 03:40:53 -- 1657408 Kbytes
Blocked:CKPT

Blocked:CKPT
Sat Oct 26 21:10:25 PDT 2013 :: Could not unblock the DB
Sat Oct 26 21:10:25 PDT 2013 :: Cleaning up, stopping DB
==== End of Log file content ===

Conditions:
This error occurs at when an Informix checkpoint is in progress at the same moment the database status is checked. There is no attempt to retry the check and the upgrade fails. Confirm the log file listed above has the specific error listed to confirm this defect is the root cause for the failed upgrade. From 10.0 on we will retry & as long as the blockage is transient (as in this case) the upgrade will still succeed, so this specific issue is corrected. Any possible blocking event that doesn't resolve within 70 seconds or so will still fail, with information in ora-install.log showing 3 successive failures before the retries are abandoned.