Rules... here's one. The east/west min times between flights rule. Supposedly not a legality but a "business rule" to minimise fatiguing patterns and timezones.
Rigidly applied when trying to swap, but totally legal when they roster less and you query it.
I get why the rule exists, but it's to prevent repeated use of those patterns, not the one off you actully want to do.
Ditto for rostering ULR after short blocks of leave, rendering 5 days pretty much useless for travel except elsewhere in the Gulf.