There's a whole nuther article about "what users don't understand about your web app"
That network/DNS/router/etc issues aren't your fault and you can't fix them.
Everything regarding passwords and account security.
URLs, esp typo's and that The Google (esp it's search bar) != Internet.
HTTPS
Phishing, and a slew of other attacks. Also that punching the monkey is probably a bad idea.
Browsers, different browser versions, different browser manufacturers, cross browser incompatibility. Also that you did not create the browser and you can't fix it. Also why they have to download install new version of browser to use your software.
Why your webapp quits working when they aren't connected to The Google.