For a brief while, all .se domains were inaccessible to most Internet users due a small one-character typo.
The official explanation is at Incorrect DNS information | .SE. What seems to have happened is that someone left off the trailing “.” in the routine republication of the official announcement of the .se root zone.
The mistake was identified within an hour or so, and the official .se data republished, but without some of the authentication information it would usually carry. The result was, if I understand what happened, was that .se remained inaccessible for a while for two groups of people: those whose ISPs had uploaded the erroneous .se data and hadn't gotten around to updating to the corrected .se info, and those whose ISPs are meticulous about validating DNSSEC signatures and noted that the (corrected) replacement failed that test.
In short, the laziest and the most painstaking were the most effected.
Update More at Sweden’s Internet broken by DNS mistake, including this:
The problems were made worse by the fact that DNS lookups are cached externally. Since DNS lookups are cached a certain time and the .se zone has a 24 hour time-to-live (the time information is cached by external DNS servers), the problem could last for up to 24 hours for some users.
Problems that affect an entire top-level zone have very wide-ranging effects as can be seen by the .se incident. There are just over 900,000 .se domain names, and every single one of these were affected.