Hello,
There are plenty of exceptions and I hope this one is an exception as well.
I heard this sentence:
"Where is mommy go?"
Normally I would have asked:
"Where is mommy going?"
Is there any reason not to use the gerund?
P.S: If you need more context, I will quickly upload the scene, no problem. 
Cheers!
No exception here Nightmare. "Where is mummy go?" is definitelly wrong, as many others have already pointed out. I just would like to add the following two remarks:
1) Native speakers do not necessarily speak only standard perfect English. They make mistakes as well. Usually their 'mistakes' are of a different nature than ESL students ones.
2) The process of communication has at least two important parts: what the speaker says and what the listener hears. The listener is already 'tuned' to English, so he has some standard constructions he expects to hear. In the listeners mind he usually fills some gaps in what he hears to match to some standard sentences he already expects.
And the same happens to the speaker, he 'thinks' he has said the words completely, and the listener fills the gaps. This is a common process in any language, not just English.
Have you ever noticed (in any language) that sometimes the listener doesn't understand something and asks for a repetition (sorry? I beg your pardon?), and before the actual repetition takes place he manages to understand? I mean sometimes you do not understand exactly a sentence immediately, but you only some seconds later.
No native English speaker is expecting ever to hear "Where is mummy go?" so he fills the gap.
