In my case it was my brain going "pick a poison, any poison--that'll be it!!" WRONG. It reminds me of a section in the Laurie Anderson song "Babydoll":

I'm wracking my brains trying to think
Of another word for horse
I ask my brain for some assistance
And he says, Huh... Let's see... How about cow?
That's close.

Poor arsenic and cyanide--so unique, each their own special poison, and people like me go and grab whichever is in the mental closet!