I also accidentally e-filed the wrong return once, two clients with the same name (why people feel compelled to name their sons after themselves...). You just have to tell the client what happened, apologize and explain that, despite rumors to the contrary, tax professionals are human.

I changed my procedure to hold 8879s and file them the next morning after I've made a cup of coffee and the before the phone starts ringing.  And after I've checked email and voice mail for one of those messages about how they forgot to tell me they got married, had a child and started a business and we may need to change the return before sending it.