Ran into this situation tonight and ruled in favor of the investigator this time (which was ultimately futile as Azathoth devoured the world as the clock struck 12 after his turn, oh well). Didn't notice a rule preventing this, but it did feel a bit fishy.

Adventure in this case was Tempest In A Teapot with the ORDERED tasks of {4 Investigation}; {Terror}; {Peril, Lore}.

First roll had sufficient investigation and the excess dice contained a Peril and a Lore. The investigator used dice to complete the investigation task, and then cast a double lock Spell, locking the Peril and the Lore. Next roll had a Terror. The following roll he used the locked Peril and Lore to finish the adventure.

I believe the rules for Focusing and Assisting are that they can't be done if the investigator has chosen to complete a task with dice, meaning he would have to roll all the excess dice the next turn. I'm not really clear how that relates to Spells and whether you can complete a task and lock excess dice when resolving a single roll. Does completing a task with the dice prohibit you from locking the excess dice from that roll? Focusing and Assisting: clearly not. Spells: well….. maybe?

I think I ruled it ok because it was coming down so close to the wire. The late game was coming up bonkers with double monsters on practically everything and the yellow die was triple-locked. It was 11 doom to 12 elders on his turn with two players ready to turn in trophies for elders right after him. Clock struck midnight; +1 doom game over. A classic finish, literally. If the rule got fudged, it certainly increased the drama to heighten the dramatic failure.

Rules (Revised at least) say you roll the dice, then cast your spell and put the dice (1 or 2) on the spell without changing the result. There is no need to fail a task to use a spell, like with assist/focus. Spells are very good when attemping arrowed Adventures.

I totally agree what Dam has said. The (revised) rule allows you to cast spells after rolling dice and put dice on them without limitations, so you don't have to fail a task in order to do so.

As a follow-up to the OP's question: suppose the first roll was 2Inv, 2Inv, Peril, Lore, Peril, Peril; the 4Inv are used for the first task and the Lore and Peril are saved on the spell as stated.

Rolling two dice results in 2Inv and 2Inv, so one die is discarded.

Rolling one die results in Terror, completing the second task.

Is it still permissible to use the two dice from the spell to complete the third task even though there are no more "free" dice to roll?

Or would the player have to play an item card (to add a red or yellow die) or unlock and roll one of the spell-locked dice in order to continue their turn?

Thanks.

I don't see why not. It's still your turn and you are attempting to complete the task; the rules you may pull a secured die at any time to complete a task, and if you're up to the last one I don't think it matters whether you rolled anything or not--you have the dice to finish available to you.