http://news.bbc.co.uk/1/hi/technology/7824939.stm
The worst ones I've seen: (some of which are listed)
Not being specified right in the first place
Not reading the (correct) spec
"." instead of "," (or vice verse)
missing {braces}
Memory leaks/buffer overrun/broken stacks
Buggy compilers and processors
Just not considering everything ("Millennium bug" was most often caused by this -Century-change/rolling over using 2 digits to store numbers which could be 4 digits long - not allowing for minutes to be up to 61 seconds long, 16-bits to store much larger values...)
Floating-point related oddities
UNTESTED CODE! - If somone would have tested the applications properly, often the bugs would be located.
