Illegal spell casting

[Octopusman]

I'm thinking of rule 717.1. Is this the right rule?

Let's say I have a City of Traitors, Mountain, and Trinisphere on the battlefield.
You have to pay for costs when casting a spell.

If I say "I'm casting Chrome Mox" without tapping anything and I place it on the battlefield because I forgot to pay 3, does the spell get reversed even though nothing was paid or do I HAVE to pay for the mox because I could? Can my opponent force me to pay for the mox because I can even though I tried to resolve the spell before paying for it?

[Edit: I think I found the answer. It would back up and I can choose not to play. I'd probably get a warning. Correct?]

[jrsthethird]

I'm thinking of rule 717.1. Is this the right rule?

Let's say I have a City of Traitors, Mountain, and Trinisphere on the battlefield.
You have to pay for costs when casting a spell.

If I say "I'm casting Chrome Mox" without tapping anything and I place it on the battlefield because I forgot to pay 3, does the spell get reversed even though nothing was paid or do I HAVE to pay for the mox because I could? Can my opponent force me to pay for the mox because I can even though I tried to resolve the spell before paying for it?

[Edit: I think I found the answer. It would back up and I can choose not to play. I'd probably get a warning. Correct?]

That's correct. You effectively tried to pay :0: for a spell that costs :3:. The game reverts back to the point just before you tried to cast the spell, You're free to either cast the spell for the appropriate cost, play another spell, pass priority, or anything in between. In Competitive REL, this is a Game Play Error - Game Rule Violation, which results in a warning.