I probably don't need to go into detail. Just say this to her:
"What is 99 centillion times 99 centillion?"
Siri recognizes the word, but she should mishear it. Edit it if you can. Be patient, her voice will eventually change.
If you want me to go into detail...
One centillion is a number, which is equal to either 10303 or 10600, depending on the system used."
Source: WikipediaIf you didn't know already, Siri uses WolframAlpha for most (if not all) asked calculations. WolframAlpha also has a website, so you may find what it looks like here:
https://www.wolframalpha.com/input/?i=What+is+99+centillion+times+99+centillionIf you do not trust the offsite link, this is the result:
Anyway, after saying the input to Siri, she should be silent for a bit, and then randomly start blurting out zeros. (It may just be my device, it's an iPhone 5) Her voice will change abruptly. I don't know if it's supposed to be the male version of Siri or a calculation error, but that's how to break Siri.
Apple listens to our conversations with Siri