These are identical fields: Drop Down (Single Select) with identical CB Conditional settings. What causes it to fail? When the value is empty. They are a drop down of our class locations each year. cb_2016class (ID: 133) and cb_2017class (ID: 136)

So you're trying to hide a dropdown field if another field is empty? What type of field is cb_2017class?

They are all Read Only, and are actually modified through my external PHP code that loads JFactory and the CB API (another one of my posts).

Does disabling the read only allow the condition to work fine? If so then it'd be a problem with how read only fields values are parsed otherwise it could be a null to string conversion problem, but unlikely.

So the field is conditioning it self? If that's the case then that's why it's being stuck in a loop. You can't do that with a jQuery based condition and would have to use a static condition (Custom > Value). The it's doing this is when a field is hidden its condition is then triggered to allow condition chaining. In this case the condition is just triggering it self over and over.