I actually had a heisenbug related to Google calendar not caring about timezones. One of our internal scheduling tools used Google Calendar as the backend but

I actually had a heisenbug related to Google calendar not caring about timezones. One of our internal scheduling tools used Google Calendar as the backend but had a custom frontend, It was to schedule a service call and the scheduler didn't actually care who made it as long as someone did at the appointed time so the frontend aggregated multiple calendars and displayed all free times. Most of the people being scheduled were in New York, but a few were in Arizona. Intermittently people in New York would complain about not being able to schedule after 5 pm EST on Friday. Refreshing the page sometimes fixed it Certain it had to be a timezone bug I put off working for it for as long as possible. tl;dr; The people in Arizona had all day blocks on the weekends. The all day events had no timezone because the calendar has it. The default timezone in the library I was using sets the timezone to GMT. 5 pm EST is midnight GMT. So after 5 PM Friday it looked like no one in Arizona had free time. It was intermittent because as a form of load balancing the service calls I had randomized which calendar I tried to schedule first. The team in Arizona was dissapointed when I finally fixed it.