This is a tough line to walk since I try not to focus on the negative. But something that gets me riled up every time is inaccessible demos showing other developers how to code. Often the message is "we're focusing on something specific, accessibility wasn't the purpose of it", or "it's just a demo", but every unlabeled form control or image missing an alt attribute teaches an antipattern and further cements people's lackadaisical attitude about accessibility basics.
Rule of thumb: if you're demonstrating anything related to a User Interface, accessibility is a fundamental requirement. Approach every demo as a teaching moment!