Is this a mistake:

La magra kato probable estas forvaginto

Shouldn't it be forvaganto because the cat is currently in the state of being homeless?

Forvaginto is correct, because the cats is currently (est'as) in the state of having wandered off at some time before now (for'vag'int'o).

Now, if the cat's owner is called Schrödinger, then the cat has wandered off and has stayed at the same time, and this system will collapse into only one solution as soon as we measure the current state of the cat. Bear in mind also, that the more exactly you determine where your cat is, the less exactly you know where it is wandering to, and vice versa. Cats are the quantum mechanic kings of the animal kingdom.

Forvaganto however means that the cat is currently committing the act of wandering off. I.e. "La kato estas forvaganto" could mean that the cat is packing its stuff, cancelling its phone line and internet service, making a last visit to the neighbours for saying goodbye, crapping a last time on the carpet, and sitting somewhat sad, but at the same time happy (cats are bipolar and histrionic by nature), on its favourite rug, pondering its past, present and future condition.

On a side note, this is a very nice example for a sensible usage of compound tenses in Esperanto.

"La magra kato forvagis." emphasizes the cat's act of wandering off. We want to express that the cat actually did something, in this case, it bailed out.
"La magra kato estas forvaginto" emphasizes the condition of the cat now. We don't care whether it walked out hitch-hiking or hijacking a car or on paws. We don't care about the act of walking out, but we are interested in the result: The cat isn't here anymore. This is to a certain degree comparable to the English perfect and to a higher degree to the perfect of Ancient greek: "τέθνηκεν" which literally means "he has died" simply means "he is dead."

Yeah, we have two cats. They are definitely quantum mechanical .

Ĉu ĝi ekvagis foren aŭ vagas fore?
Is it that it "ekvaĝis foren" or "vagas fore"?

Edit: Tekstaro has: "La maljuna hundo dum tuta tago vagadis longe for en la urbo."

If you want to describe the cat as a stray, then forvaginto.

As Nornen says this is viewing the cat as at some time having wandered off.

The use of -ant and -int is all a matter of perspective - how you see the action.

Would you describe yourself as a komencanto (though at one time vi komencis) or as a progresinto (though you may still be progressing).

The former president of your local club is technically a prezidinto, but you are more likely to describe him as an eks-prezidanto. And a former lover is going to be an eks-amanto.