Go pear shaped (The plan isn't working / plan is going wrong)
... In all likelihood, this has nothing to do with the fruit, but rather plays off of the expectation of something being round, but instead having it be an odd shape.
Could I have your help in interpreting the phrasal verb? I looked it up in dictionaries. It has several meanings. I expect that means 'to react to or interact with, as in a drama'. I'm not sure of whether I am right though.