What is the Best Bug-as-a-Feature?

Bat Country wonders: "The workflow system, at the department I develop for, was hand-coded by my predecessor in a rather short amount of time, resulting in somewhat unreadable code with a number of interesting 'features.' When I took over maintenance of the code base, I started patching bugs and cleaning up the code in preparation for a new set of features. After I was done, I got a pile of complaints about features that had disappeared, which turned out to be caused by the bugs in the code. So, that leads me to ask: what is your favorite bug that you either can't live without or makes your life easier?"

Comments

buffer overflow

[virtualXTC]

buffer overflows are great - they allow you to get root on all sorts of devices that some bastard tried to lock you out of.

ModeX graphics? Buffer overflows?

[Kufat]

The famous undocmented 320x240 VGA video mode, pre-VESA, and other tweaked VGA modes.

I've heard the 6502 (or, more specifically, RP2A03) had some useful undocumented opcodes. I think they weren't intentional, so they might count.

On the software side...how about exploitable buffer overflows on the Xbox and PSP to enable execution of arbitrary code?

Re:Yes

[pilgrim23]

though Windows does this with flying (Blue) colors, I would nominate DRM. it screwed music and spurred development of P2P

Re:Perl versus Python

[zippthorne]

As an engineer, I'm also amused.

4.0 is definitely a 2 sig dig number, but 4 could be 1 sig dig or it could be "exact." You wouldn't say the 2 in 2*pi*r is only one significant digit, would you? Of course not, it would render any precision in pi or r meaningless.

Also, 1200 could have 2, 4, or be exact depending on the context. It's best to always use a notation that includes the fraction mark for non-exact quantities.

Ah, Units

[EgoWumpus]

As a programmer, and a former student in at least one other math-related discipline, it's clear to me that 4 and 4.0 are equal. But they are not equivalent. Knowing 'Accuracy', 'Precision' and 'Proper Use Of Units' like the back of your hand will help you in any career.

Re:rm

[cswiger]

Nice. Perhaps having rm look at a filename called "-i" and interpret that as a command-line flag is probably the best "bug as feature" offered so far. :-)