You can either use bugtoolkit (requires CCO account) or read the releasenotes to find out about caveats in IOS releases.
For instance if you have 12.4(6)T installed, you can read about caveats that where fixed in 12.4(9)T but affect the current release.
It useally has a section with open and resolved caveats. If the particular technology affects your IOS you can decide to upgrade.
If a bug has a big scope or if many bugs affect a certain release Cisco declares it as a "deferred" (DF) release.
HTH
--Leon
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