The highest profile incidents have been those involving EFTPOS machines in Australia and elsewhere. There's no question that this wasn't a real - and serious - bug.
Also coming in for significant attention was an alleged 2010 bug in the SpamAssassin 3.2.x spam filter.
One of the ways SpamAssassin determines the spaminess of a particular message is to check its date. If it's too far into the future, the assumption is that the message is likely to be spam.
While a message apparently from the relatively distant past is possible (typically due to a dead battery in the sending hardware combined with disabled network time setting), messages apparently from the future are generally engineered that way so they are forced to the top of the receiver's inbox.
SpamAssassin's behaviour is controlled by a configuration file, and one of the rules controls the threshold year used in calculating a messages spam score.
Up until the end of 2009, that threshold year was 2010. On January 1, a replacement rule was pushed out to installations using sa-update (Spam Assassin Update).
That was least a day late, and admins in parts of the world where it is customary to take time off between Christmas and New Year would likely wish it had appeared at least a month eariler.
Not everyone uses sa-update, so some manual configuration was required. (That also went for Mac OS X Server admins - it seems Apple did not push out the new rule via Software Update.)
The rule had already been changed in SpamAssassin 3.3.x.
Bug or not a bug - see my argument on page 2, then leave a comment with your opinion on the matter.
But the question is, was this a bug?
You can argue that there is a bug in the program's design in that it should use a certain number of years into the future rather than an absolute year as the threshold, but that's a different matter.
It's not that SpamAssassin couldn't cope with dates falling in or after a certain year, which was at the root of the EFTPOS problem.
Analogies are always tricky, but it's a bit like using an address book program to manage your Christmas card list, and then complaining that one of the labels is wrong because you didn't enter a friend's change of address.
If you think the SpamAssassin issue really was a bug, here's an advance warning for you: SpamAssassin now either has a bug in that it no longer uses future dates as an indicator of spaminess, or has a 'Y2K20 bug' if you use a rule like the one suggested by Apple .
Some SpamAssassin users suggest that checking for future dates does nothing to improve the accuracy of filtering so it might as well be deleted entirely. They say that while virtually all messages with distant-future dates are spam, those same messages are detected by other rules.