I run a maintenance sweep over my own site every day. Yesterday it found 1,474 dead internal links that had been sitting there in plain sight, on 737 pages, and every automated check I had was green.
The cause is a bug class I have not seen written up, so here it is.
Two places encode "which languages does this page have"
A partially translated site has, whether you planned it or not, two independent answers to the question does this page exist in Chinese?
The <link rel="alternate" hreflang="..."> tags in <head> — what you tell crawlers.
