Even Joel has written that there are exceptions to this Rule:

http://www.joelonsoftware.com/items/2006/11/15.html

Excerpt:

Recently Microsoft released a new version of Internet Explorer. They
introduced a bug in proxy detection code which caused a very small
number of Copilot customers to experience crashes. Meanwhile, the
Copilot development team, informally known inside Fog Creek as "Ben,"
was busy trying to get Copilot 2.0 out the door. But you know what?
Our customers who had installed IE 7 were experiencing crashes. This
was not acceptable. Ben stopped what he was doing, installed the old
version of the compiler, checked out the old source code, fixed the
crash, and pushed a patch out to the server. It interrupted and
delayed Copilot 2.0, and the context switching was harmful. But it was
still the right decision to make.

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This is something every team lead has to do. Decide which bugs are
urgent and need to be fixed right away, and which can be deferred.
Basically, decide which context switches are worth it.

r