Considering the problems it can cause for some users, why does Apple release public betas of its operating systems? Because there's only so much that can be done in a development lab--bug testing needs to be done on a large scale. (You can also say there are marketing reasons, but that's for another discussion.)
If you're ready and willing to use Apple's beta operating systems, that's great. You get to try out the newest features. But while you're busy pinning sites to Safari 9, perusing the News app in iOS 9, or splitting the screen to your heart's content, and your computer crashes or exhibits some other oddity, please report it to Apple. Remember what the purpose of the beta is for, and take a moment to let Apple know what went wrong.
On the El Capitan public beta, you report issues through the Feedback Assistant app, and iOS 9 has a Feedback app too. When you install one of these betas, the app is automatically installed, and El Capitan's Feedback Assistant app even auto-launches as a reminder to file reports. But I bet a lot of you quit the app and went along your merry way. So, if you're unfamiliar with Feedback Assistant, this article will walk you through the steps of using it.
Using Feedback Assistant in El Capitan
To launch Feedback Assistant, go to /Applications/Utilities. You'll find an alias of it there. It actually resides in /System/Library/CoreServices/Applications, but there's no need to navigate there when you can just use the alias.
Once you launch Feedback Assistant and continue past the intro screen, you need to log in using your Apple ID. It needs to be the same ID you used to register for the Apple Beta Software Program.
After you register, you'll see an interface that looks a lot like the Notes 4 UI in El Capitan. To report an issue, click on the New Feedback button icon button next to the search bar in the second pane, or click the New Feedback button in the viewing pane (if you have no feedback selected or have selected the welcome message), or type Command-N, or select File > New Feedback.
When you create a new feedback report, you have to fill out a form. Click on the Help button on the lower left corner (the "?" icon) for guidance on how to fill out the form. You can even file reports for the iOS 9 and Yosemite betas by clicking on Change in the upper right and selecting the OS you want.
Don't be afraid to be detailed in your report. Writing the specific steps you took to produce the problem. Explain what you thought would happen and what actually did happen. Note down any error messages. It's more important to explain your experience than to try and provide your own diagnosis on why there's an issue. And don't think that someone else has probably already filed a similar report; multiple reports of the same issue can help raise awareness for the OS developers.
Once you filled out the report and click Continue, you can add any files that may help the developer, like screenshots. Feedback Assistant will attach a system profile and a compressed folder of diagnostic files.
In the final screen, you can review your report. Click Submit when you're done. Congratulations! You did your duty toward building a better OS. Keep up the good work!
Using the Feedback app in iOS 9
Before reporting a a bug in iOS 9, take a few steps to see if the problem is reproducible. Say you're having trouble tapping the tabs in the Twitter app. Try force-quitting the app (double-tap the Home button to get to the app switcher, and then swipe up on the Twitter preview to quit the app) and relaunching it to see if that helps. Ditto with restarting your phone. It might not help, but at least you'll be able to say in your report what fixes you tried.
If the problem is with a third-party app, check the App Store's Updates tab to see if there's an update for the app in question--no point submitting feedback for a bug that's already been fixed. You can submit a screenshot with your report, so if the bug is visual, go ahead and snap some proof! This won't make sense for every bug, of course--it's pretty impossible to get a screenshot of an app crashing.
The Feedback app is pretty bare-bones, opening to a Feedback Assistant screen with the same kinds of folders you'd see in an email app: Inbox, Drafts, and Submitted. (Inbox is where Apple can leave notes and announcements for its beta testers, i.e. you. Right now all that's in there is a welcome note.) To start a new report, click New Feedback, or the compose button in the top-right.
Then you're basically just filling out a form. Be descriptive and thorough as you answer each question. In the Basic Information section, for example, you'll be asked for a descriptive title. Be specific! Don't just call it "Bug" or "Crash." Instead, flesh it out a little to something like, "Apple Store app crashes on launch."
At the end of the form, you'll get a chance to add a screenshot or video, and if you'd like you can click the crash log attachments that are added automatically to give Apple's engineers a more complete picture of what happened. After a reminder that the logs are being attached, you'll tap Submit in the upper-right. Then pat yourself on the back for doing your duty as a beta tester!