Joel Kallman had an interesting post about a customer who broke all of the rules and modified some constraints in the APEX_030200 schema. In this case, there was a happy ending, as Joel went the extra mile to craft a custom patch to remedy the issue. However, things don't always end up this way. This reminds me of my early days with Oracle, when I used to work with the eBusiness Suite. The first page of any documentation of the suite said something like this: "WARNING: Do not use SQL*Plus to modify any database object directly". We used to laugh at this, as we used SQL*Plus & TOAD to modify these database objects several times a day, if not more frequently. However, this was done on internal systems used to build demos. If we messed something up - and boy did we ever - we could easily refresh the instance with little effort. Fast-forward a few years to today: FND & APPLSYS have been replaced by a single, much smaller schema called APEX_...
Mostly Oracle APEX. Mostly.