Here's a quickie: when you are documenting something, or adding help text to an input, always double check your examples.
I just created a date picker and wanted to set the minimum date. Since I was working quickly, I glanced at the examples in the help text, which says,
Enter a minimum selectable date. The value can be absolute value in YYYYMMDDHH24MI format or relative value with respect to today's date e.g. +7d ('y' for years, 'm' for months, 'w' for weeks, 'd' for days) or name of the item, which holds the value e.g. &ITEM_NAME..
So, I punched 01-DEC-2010 into the field, saved, and ran the page. If you're reading closely, you won't be surprised at the error I got (though I was):
Error ORA-20001: Entered Minimum Date attribute value 01-DEC-2010 for item P55_RESOLUTION_DATE does not match YYYYMMDDHH24MI format.
Yep, the very first example in the help text uses the wrong format, and APEX doesn't catch it until it tries to render the page. Changing to 20101201 (note that you don't need the time data) resolved the issue.