This article is from the Macintosh system software FAQ, by Elliotte Harold firstname.lastname@example.org with numerous contributions by others.
A Type error is your Macintosh's way of telling you that it's
sick and plans to take a nice vacation in Belview for a few days.
Among developers Type errors are officially known as DS errors where
DS stands for "Deep Spaghetti" (though a somewhat more colorful
expression is often used in place of "Spaghetti"). Your
applications are toast. Any unsaved data is lost. Once you've
been hit with a Type error there's absolutely nothing you can do
about it. You'll probably need to restart your Macintosh either
by hitting the programmer's key or by turning the Mac off and on
if the programmer's key isn't installed.
The most common type errors are Type 1 and Type 3. Type 1 is
a bus error. It's most commonly symptomatic of software that isn't
32-bit clean. A Type 3 error is an illegal instruction. It's most
often symptomatic of poorly written software. You may occasionally
be able to avoid Type 1 errors by turning 32-bit addressing on or off
or by turning the cache on or off if you have a 68040 Mac. Otherwise
there is almost nothing you can do about these errors except try to
find out what actions, applications, and/or extensions cause them
so you can report them to the programmer and avoid them in the
future. There is no point posting about Type errors to the net.