Title text:
Other numbers are largely unaffected, but anyone using '4' for their certificate key is urged to upgrade to '5'.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3314/
"Half six" is fraught with peril. One slight misunderstanding and you'll have Germans and Dutch at your door when you least expect it and a bunch more people an hour later.