The program-design version of the law of diminishing returns. The 80/20 rule says that roughly 80% of the problem can be solved with 20% of the effort that it would take to solve the whole problem.
For example, parsing e-mail addresses in "From:" lines in e-mail messages is notoriously difficult if you follow the RFC 2822 specification. However, about 60% of actual "From:" lines are in the format "From: Their Name
(Adding support for "From: user@host" and "From: user@host (Their Name) " brings coverage to almost 100%, leaving only really baroque things that RFC-2822 permits, like "From: Pete(A wonderful \) chap)
Compare: KISS Principle