What is the meaning of the following sentence?
You have successfully split a hair that did not need to be split.
Source: this post on the Programmers Stack Exchange.
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To "split hairs" is to be overly pedantic and precise (see http://www.merriam-webster.com/dictionary/split-hair). Someone saying that you split a hair that didn't need to be split is going one step further, saying that not only were you overly precise but it was well beyond what was needed.
To split hairs means
to pay too much attention in an argument to differences that are very small and not important
This often occurs as "let's not split hairs," meaning "let's not argue over trivial differences."
According to Wiktionary, the plural split hairs is also used as a noun, to mean
Tedious details; minutiae.
In your example, the writer takes it one step further by using split hair as a singular noun meaning a detail or difference. This use is probably not as common, but it's understandable.
So, "You have successfully split a hair that did not need to be split" means "you have managed to point out a small difference where it was unnecessary to do so; you have made a distinction where none needed to be made."