The idea that you can "accidentally" perform a complex secret Dark ritual is ridiculous. Especially because accidental magic has previously always matched the goals of the caster, but Voldemort didn't want Harry to live and certainly didn't want to make him into a living Horcrux (although that would have been cool if he actually did, and had planned that). Also, this seems to be incompatible with how HPMOR magic has already been established to work so EY will need to fix this one.

Eliezer has deviated from canon in such a way that it allows the method for casting dark rituals to be congruent with what happened on Godric's Hollow. We are still missing a lot of information on what Voldemort's intention really was, and if he did intend to kill Harry at all, but there has been speculation on previous threads that Voldemort, in his hubris, accidentally fulfilled the requirements for a certain ritual.

From Chapter 74:

Professor Quirrell's lips twisted further. "Ah, but the truly amusing thing was this. You see, Mr. Potter, the chant of every ritual names that which is to be sacrificed, and that which is to be gained.

And previously on Chapter 43:

"I give you this rare chance to flee," said the shrill voice. "But I will not trouble myself to subdue you, and your death here will not save your child. Step aside, foolish woman, if you have any sense in you at all!"

"Not Harry, please no, take me, kill me instead!"

The empty thing that was Harry wondered if Lily Potter seriously imagined that Lord Voldemort would say yes, kill her, and then depart leaving her son unharmed.

"Very well," said the voice of death, now sounding coldly amused, "I accept the bargain. Yourself to die, and the child to live. Now drop your wand so that I can murder you."

So in this interpretation, Lily Potter's life was to be sacrificed, and Harry's to be gained. What possible side effect would happen when/if Voldemort tried to murder Harry, we can only speculate.