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Sure, you might be right.
Today i've been playing more and it may be just a bug in the spanish version. Have been playing in English and not being able to choose Saga.

Cavern can still produce colorless mana. It can be used for generic costs, but doesn't make the spell uncounterable. So you probably had the colors available already and the Cavern just gave you the generic mana.

The funny thing is, I'd swear I was able to cast Sagas with Cavern yesterday. But I agree with what you're saying: if Saga is an enchantment-only subtype, you shouldn't be able to choose it with Cavern as a creature type.

You find Saga only in the enchantment type list, not the creature type list. That's why I asked how you could choose Saga in the first place. Since you could, that is the bug.

205.3h Enchantments have their own unique set of subtypes; these subtypes are called enchantment
types. The enchantment types are Aura (see rule 303.4), Background, Cartouche, Case (see rule
719), Class (see rule 716), Curse, Role (see rule 303.7), Room, Rune, SAGA (see rule 714),
Shard, and Shrine.

205.3m Creatures and kindreds share their lists of subtypes; these subtypes are called creature
types. One creature type is two words long: Time Lord. All other creature types are one word
long: [...] Rabbit, Raccoon, Ranger, Rat, Rebel, Reflection, Rhino, Rigger, Robot, Rogue, Sable, Salamander, Samurai, Sand, Saproling, Satyr, Scarecrow, Scientist, Scion, Scorpion, Scout, Sculpture, Seal, Serf, Serpent, Servo, Shade, Shaman, Shapeshifter, Shark, Sheep, Siren, Skeleton, Skunk, Slith, Sliver, Sloth, Slug, Snail, Snake, Soldier, Soltari, Spawn, Specter, Spellshaper, Sphinx, Spider, Spike, Spirit, Splinter, Sponge, Squid, Squirrel, Starfish, Surrakar, Survivor, Synth, Tentacle, Tetravite, Thalakos, Thopter, Thrull, Tiefling, Toy, Treefolk, Trilobite, Triskelavite, Troll, Turtle, Tyranid, [...].

Because the game let's you choose saga as creature type... If Saga is not a creature type, the game must not let you choose it.

How did you even choose "Saga" as the type? It is not a creature type and thus you should not have been able to choose that in the first place. So if anything, choosing "Saga" is the bug. ("Saga" is an enchantment type.)

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