No matter how well you prepare a talk, you can always have technical difficulties. If you handle the failure gracefully, the public won't mind.
Here's a video of a talk I did this year at PHP Experience 2018 in Brazil. The plan was to showcase the boot sequence of the Raspberry Pi starting our dashboard. I mistakenly packed my kids' Raspberry Pi with their 16 bit games instead of the Pi containing the dashboard. 🤦♂️