The Knights of Noromath server observes the following pen and paper conventions with respect to multiclassing.
Oathbreakers and Changing Your Religious Path
Generally speaking, there are serious penalties in store for characters that break their religious commitments with respect to the class levels they pursue in game. While there are some exceptions to these conventions based on the worship of specific deities, characters risk becoming one of the following "Ex" classes if they fail to adhere to their sworn path.
Ex-Clerics - Being an ex-cleric means that you've grossly violated the code of conduct required by your god. You lose all clerical spells and related specialty priest benefits if any.
Ex-Monks - Being an ex-monk means that you've left the training of your temple or lost your lawful alignment, whatever the case may be. You can never again advance as a monk, although you retain your abilities.
Ex-Paladins - Being an ex-paladin means that you've left the training of your order or lost your lawful good alignment, whatever the case may be. You can never advance again as a paladin, although you retain your abilities.
Fallen Paladins - Being a fallen paladin means that you've deliberately committed an evil act or grossly violated your code. You lose all your paladin spells due to your transgressions and you may never advance again as a paladin.
Fallen Druids - Being a fallen druid means that you've ceased to revere nature, changed to a prohibited alignment or taught the Druidic language to a non-druid. You lose all your druid spells due to your transgressions and you may never again advance as a druid. (Unless you become a Blighter)
Deity Related Exceptions
Characters who worship a deity in the temples of Faerun have access to special training as a benefit of their organizations.
Azuth - Multiclass Freely as Wizard
Ilmater - Multiclass Freely as Cleric, Divine Champion, Ranger, Shadowdancer
Lathander - Multiclass Freely as any single other class
Selune - Multiclass Freely as any single other class
Sune - Multiclass Freely as any single other class
Azuth - Commonly multiclasses into Wizard, still becomes an ex-paladin.
Helm - Multiclass Freely as Fighter, Cleric, Divine Champion, and Cavalier
Ilmater - Multiclass Freely as Cleric, Divine Champion
Kelemvor - Commonly multiclasses into Cleric or Ranger, still becomes an ex-paladin.
Milil - Multiclass Freely as Fighter
Mystra - Commonly multiclasses into Wizard, still becomes an ex-paladin.
Sune - Multiclass freely as Divine Champion
Torm - Multiclass freely as any single other class
Tyr - Multiclass freely as Cleric, Fighter, Divine Champion
Some special exceptions exist such as the following:
Halfling Paladins - Arvoreen's Marchers, Multiclass Freely as Clerics, Fighters, and Rogues
Dwarven Paladins - Multiclass freely as Clerics, Fighters, Divine Champions, and Dwarven Defenders
The following classes require a milestone to be reached in order to advance beyond a certain level. A roleplay milestone is simply an event or incident that must transpire in order for a character to proceed on a path. Any DM can provide Milestone Roleplay, and in some cases players may provide milestone roleplay for eachother.
Blackcoat: This class requires DM approval or PC tutelage to pursue. (Level 1 locked)
Blackguard: This class requires a roleplay milestone to be reached to advance beyond class level 4.
Bladesinger: This class requires DM approval or PC tutelage to pursue. (Level 1 locked)
Druid: This class requires a roleplay milestone to advance at all in levels 11-14, but they can multiclass into other classes without penalty, remaining druids so long as they maintain their reverence for nature.
Red Dragon Disciple: This class requires DM approval to pursue. (Level 1 locked)
Warlock: This class requires a roleplay milestone to be reached to advance beyond level 4.