PHP Error if Serialized Data Corrupted

I was doing an upgrade of an existing site from what I assume was a very old version of 6.x-1.x-dev (core was Drupal 6.9) to 6.x-1.0. The upgrade appeared to have worked.

Looking at dblog a little while later I noticed PHP errors from headerimage at line 186. Tracing the problem I discovered I had about 6 nodes with bad serialized data (looked ok to me, but unserialize failed trying to decode them). Since unserialize couldn't handle them, it returned ia null result. That generated an error in the foreach($conditions...) on line 186.

My solution was to surround that foreach with an if (is_array($conditions)) {.......} test to keep the error from happening.

And then I manually edited each of the bad nodes, where all fields came up empty but I could deduce the values from the serialized data, and clicked save to fix the bad serialized data. I have no clue what cause the serialized data to be rejected by unserialize.