kega no komyou
The great achievements of an injury... Sounds kind of sadistic doesn't it? Maybe it's some kind of martial-arts related phrase about the efficiency of the hammerfist technique when applied to the Triple Warmer-23? Maybe not.
Rikai-chan will get you closer, by telling you that 怪我の功名 means a "lucky hit," but this doesn't capture the nuance of the phrase. 一刀両断-ing a watermelon with a broom stick while blindfolded is a "lucky hit (or at least, it appeared to be). It's not 怪我の功名.
1. A fortunate mistake
2. A happy accident
3. Fortune disguised as misfortune
To qualify as a lucky hit of the 怪我の功名 variety, you have to start with something that seems to be bad, a mistake, a screw-up, or a calamity of some kind.
One of my books, 小学四字熟語・ことわざ has this great picture of a baseball player who has accidentally knocked a man unconscious with an out-of-the-park smash... but the black mask that the injured party wears, the sack of loot he carries, and the elated police officer standing over him reveal that this unlikely head trauma was actually a civil service!
A lot of the usage on the internet is related to cooking. "I mistook oyster sauce for ソース, but it came out DELICIOUS," etc.
Can you think of any examples of things that would count as 怪我の功名? I'll start us off with some historical examples of fuck-ups with fantastic consquences.*
Crap! I dropped all these chocolate chips in the cookie batter. Now, it's just RUINED!
... Maybe you wouldn't be so clumsy if you didn't always wear those stupid gloves when you baked...
This white wine has all kinds of bubbles in it! You've really messed it up this time!
And on top of that, it seems to be on fire...
And I'll make the last example, today's
Scooby and those dudes manage to mess up in a way that somehow catches the criminal, EVERY TIME! If you're not a little kid, it's pretty fricking boring...
*Historical examples do not accurately reflect history.