It's typically 'SESSION' specific things with connection pooling that I've seen underlying (sometimes long-uncaught) bugs in production systems.

e.g. A 'ALTER SESSION'/'NLS_DATE_FORMAT' commands in Oracle or even an unfortunate 'USE <db>' w/MySQL.

There are of course safe solutions & techniques for this, but when you have an otherwise stateless-by-design codebase (such as with PHP), picking up "possibility state-laden" connections is a bit of an unexpected concept that I've seen catch developers more than once.