Imagine a program that parses log files for the previous day and generates a summary report by hour of day and writes that to a database. The key of the table is a datetime field, and that works well because you'll only ever have one record for any given day and hour.
Now imagine that it stops working on Halloween. The error says, "primary key constraint violation." So basically it tried to write two rows with the same date and hour. What would be your very first thought as to the problem?
Sadly, I'm not that smart. I looked through the code. Traced through the code. Had it write various debugging info. Put in some exception handling and had it dump the data it was trying to write when the error occurred. I saw what it was doing - trying to write "10/30 1:00" twice in a row. But I couldn't for the life of me figure out where the bug was that caused that to happen.
Then I noticed that the clock on the wall was an hour fast because nobody had adjusted it yet for daylight savings time.
Log files should really use GMT. You know? I wasted a good 4 hours on something that should have taken 30 seconds.