Developers, in general, are very optimistic about the code they write. This is likely one cause of their estimates of the time required being low, as well as the various bugs that slip through because of corner cases that appear for the problem being solved. Often developers think they've considered the various ways this code ought to work and covered all the possibilities. Usually we find they've not thought about the problem from other perspectives and need to adjust their code.

They often also feel that their code is superior to others, and that they can examine a problem in a new way. One of the reasons that I think many developers want to rewrite systems in some new technology or new way, embracing the Not-Invented-Here way of looking at other people's code. They want to write their own solution.

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