The choice of searchers for the two standard text-based custom fields has changed in 5.1.7 and 5.2. When you create a text custom field there used to be an Exact Text Searcher and a Free Text Searcher. Now there is just the Exact Text Searcher.

Similarly single select custom fields no longer have the option of a Single Select Searcher, only the Multi Select Searcher.

Changing the searcher types for these fields changes the operators available in JQL searches. All my saved filters with clauses such as:
MyCustomTextField = 'ABC'
now need to be replaced by
MyCustomTextField ~ 'ABC'

I marked this as Critical because I don't want to have explain that most of a customer's saved filters were invalidated by an upgrade.