A cleric casts confusion on a Stone Giant and it fails the saving throw. A paladin then casts command with the one-word command, "Grovel". What happens? Some possibilities:
Spells only factor in potency when they are the same (emphasis mine):
The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect - such as the highest bonus - from those castings applies while their durations overlap.
Instead, both spells affect the target regardless of possible potency metrics such as timing, spell level, or spellcasting modifier:
The effects of different spells add together while the durations of those spells overlap.
This means that regardless of your command...
An affected target can't take reactions and must roll a d10 at the start of each of its turns to determine its behavior for that turn.
Any other activity (such as falling prone) would be considered behavior, and as such could not be done if the d10 roll decides a specific behavior before the creature has the opportunity to act on your command. Command then ends if the creature is unable to Grovel.
If the target can't follow your command, the spell ends.
Confusion takes priority because it takes effect at the start of the turn, before the creature can decide it's own behavior (and thus decide to follow command). If multiple effects trigger at the start of your turn, the GM would have to decide the order unless there is specific guidance on the text. In this case, though, command only specifies "on its next turn", so the more specific trigger has priority.
On a roll of 1-8, it's behavior for that turn is defined by confusion. It doesn't have an opportunity to fall prone since that would be a different behavior.
On a roll of 9-10, while "acting normally", the creature will fall prone and end its turn per the command.
... Xanathar's Guide to Everything (XGtE) provides a suggestion as guidance in the Dungeon Master's Tools section:
If two or more things happen at the same time on a character or monster’s turn, the person at the game table — whether player or DM — who controls that creature decides the order in which those things happen
I worked from the assumption that confusion triggers first, but I've seen arguments that the effects are simultaneous despite the differing wording.
Under this ruling and using XGtE's advice, the GM would directly decide which happens first (the creature falls prone and ends it's turn or rolls for confusion) since the GM controls the NPC.
The rules don't say with any clarity what happens when a creature is under two contradictory spell effects, so it's up to the DM to determine.
There are strong arguments in both directions, depending on how you read the spells:
If you read command as forcing the creature to follow your order with a degree of intentionality (as it has to hear and understand, and won't harm itself) and confusion as just scrambling its brain, then it makes sense that the creature acts under confusion, and only if it would otherwise "act normally" -- that is, if the creature's will actually matters -- does it follow the command.
On the other hand, if you consider that confusion is based on the creature's mind being "twisted" and reacting to "delusions" (as the spell description says), it's reasonable to rule that command overrides that, because your magically-enforced command doesn't care what the creature thinks it sees and its emotional responses to those delusions; it simply must obey. For example, if the creature were confronted by an enemy soldier and you commanded it to kneel, it would obey despite the danger. The situation doesn't change if the 'enemy soldier' is actually their ally and it's their own delusions that mark it as 'enemy'.
