I started receiving an error message via email from my Zapier zaps that talk to the YouTube API, stating that "the request cannot be completed because you have exceeded your quota." Here's the full message sent to me via email.
Error Message: YouTube: Trigger partner failure: The app returned "The request cannot be completed because you have exceeded your <a href="/youtube/v3/getting-started#quota">quota</a>.".
After I found this thread on Zapier's forum, I realized there wasn't anything wrong with my YouTube API's quota but Zapier's.
They're probably making more requests than their API limit allows, but the errors are being thrown into users as they happen when users' zaps try to fetch new content from their channels and playlists.
The good news, I guess, is that zaps aren't failing.
According to Zapier, "Zapier generates an error any time we detect something wrong with the way your Zap is expected to work. This could be a problem with the way your Zap was initially set up, or related to a specific App."