I just discovered a silly, but slick way to validate if the web.config file itself is having parse issues....

Open IIS for the site and open the Logging module/screen. If the web.config cannot be parsed, you'll get an error of this flavor:

This lead me to realize that I had a silly definition issue with my SlowCheetah transformation that has adding the url rewrite rules node twice.

Hopefully that will help someone not spend the ridiculous amount of time I did (once again) staring at my web.config and not seeing what was going on.