YouTube.com, and by extension the YouTube API, will be in partial read-only mode on Thursday, September 27 starting at 2:00pm PDT and lasting until 7:00pm PDT. Unlike full read-only downtime, this partial downtime will affect a different subset of accounts on a rolling basis until its complete. The downtime for any given account during that period is expected to be around 15 minutes.
YouTube API calls that create or update metadata or videos will fail during this period of time. As a best practice, we recommend making sure your code gracefully handles service errors on POST/PUT/DELETE operations:
-Jeff Posnick, YouTube API Team