Client: This web app you created doesn’t work!  We get an error message and then a blank screen. I’ve attached a screenshot, please use it to figure out what’s wrong.

I open the screenshot. Fair enough: it’s of a blank screen. 

Me: Can you take a screenshot of the error message?

Client: What’ll that tell you?

Me: What’s wrong. 

Client: What’s the first screenshot tell you?

Me: Your screen’s resolution.