DST change reminded me of that time QA discovered that, if you build a date from NSDateComponents, any missing items will not be set to zero, but will be set to the corresponding value from the current date and time.
Which meant that my March 30th, 12:00 would get random one-minute-drift like 12:00:59 because the user entered the date right before a minute change.
Makes sense for the date, but was a bit bewildering for the time, especially seconds.