I've been in a situation at work recently in which we've had to provide a test plan to our customer. This isn't a big deal; we're happy to provide our test plan. Interestingly, we sent this to project management for review before it went out, and got a bunch of feedback that wording should be changed in various areas to make the test plan less scary to customers. Here's the thing:
The test plan is supposed to be scary.
Your test plan describes all the nefarious things that you will do to the software before those nefarious things can happen in the field. You're going to put it under heavy load to see where it cracks. You're going to subject it to repeated hardware failures. You're going to feed it bad parameters and missing parameters. Sure, you'll test the happy path, too.
Your test plan teaches your software and your developers just like training teaches a soldier. You certainly hope these things don't happen in the field, but you train for them just in case.
Make sure your test plan looks like this:
Don't let it look like this:
For better or for worse, a scary test plan is comforting to your customers. It tells them that you're prepared for and thinking about when things are going as expected, but also that you're prepared for when things don't go as expected. Life happens and sometimes it's scary; let your test plan prepare you for it.