After years of no problems with their site, my client was suddenly unable to
login to their site using Contribute 3. Contribute would crash during "Updating
Templates" screen.

The problem turned out to be a rel="stylesheet" in an absolute link to a style

This crashes Contribute 3:
<link href="" rel="stylesheet"

This does not:
<link href="" type="text/css">

The doc type was:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"

And the stylesheet link is in the <head>.

Go figure.

Hopefully this posting will spare someone else the headache.