Sometimes I'll forget about one or more of these tools when building a course, so I thought I'd list them here as a reminder for others, too. Use all of these tools both while you are building your Canvas course and after you build your course. I've also included some tips at the bottom to reduce the number of errors and technical issues with your Canvas course.
1. Course Link Validator
Especially when you are working on more than one course at a time, you may copy and paste or share things between different courses. This may cause links or images to break for students if they link back to a different course that you have access to but not the students. You may re-use an older course which has links that no longer work. You may accidentally link to a page that you forgot to publish. The Course Link Validator in Canvas will check for issues like these. You can access it by going to your course Settings and then 'Validate Links in Content.' See How do I validate links in a course?
2. Student View
By default when you create and add things such as pages, quizzes, assignments, and discussions, they are not published. That means students will not be able to see them, even though you can. Modules with unpublished items are considered to be in a "Draft State." See How do I use Draft State in Assignments?
You may also set requirements and prerequisites or unlock dates for modules or availability dates for assignments that inadvertently block access for students at the wrong time. Again, you will be able to see and access the modules and assignments just fine, but your students will not.
You've spent hours making your course look beautiful. Lots of images, tables, videos, and other interactive features. Hopefully early on in the process, however, you regularly check what your course looks like in the Canvas app for Android or iPhone/iPad. You may be shocked to see that that table or image you used has made the rest of the page shrink down to microscopic levels. Those image buttons that lined up so perfectly on your computer are all out of whack or barely visible in the app. That Flash widget you added to a page may not even be visible in the app, and that video may be so tiny as to be unwatchable.
Your school may have UDOIT or Blackboard Ally installed, which can check your entire course for accessibility issues.
As an alternative or supplement to these tools, test out your course with a Screen Reader such as NVDA or Claro Read for Chrome or PC. Officially supported screen readers for Canvas are listed on this page: Accessibility within Canvas
Reducing the Number of Errors before They Happen
Modules - Use the modules page as the primary place where you build and organize your course. Think of it as the table of contents, or outline or to-do list, for your course. If you have a reading or assignment or quiz or discussion for a particular week or unit, add it to the module for that week or unit. Don't only link to your activities and resources within a page. This way, everything associated with that week or unit will be more visible to you and your students. You can see in a glance if something is not available or unpublished that shouldn't be, or if a requirement was not set, and so forth. See How do I add a module? and How do I add assignment types, pages, and files as module items? You can still make a nice looking homepage and module introduction pages for your course, especially if you do not like the visual appeal of the modules page. See How do I change the Course Home Page? and How do I set a Front Page in a course? Adding text headers to modules can also improve the visual appeal and readability of a module: How do I add a text header as a module item?
Tables - When inserting a table, always set a header row and/or column in the table properties, for accessibility purposes and screen readers. I would recommend never setting the width or height of the table to a fixed value. If you dragged to resize a table, then it set it to a fixed width. This will end up looking very bad in the Canvas app. As an alternative, either keep the width property empty, or set it to a percentage value like 100% or 80%. I would recommend reducing the number of columns, also, if you use tables at all. More than four columns become very small on a mobile device. See this article for information about the different table properties you can set: How do I insert a table using the Rich Content Editor? Go to 'row properties' to set a row as a header row.
Videos - When inserting videos, make sure there are captions (and ideally a transcript), and also check that in the Canvas app, the video plays full-screen, or at least that it is not so tiny as to be unwatchable. You can use the 'public resources' tool, if available, to embed youtube or other videos. See: Embedding Content Using the Public Resources LTI Otherwise if you paste in a Youtube link or use the chain link icon, see the 'alt text for inline preview' information also on this page: How do I link to a YouTube video in the Rich Content Editor?
Flash - Just, don't do it. Check that any widgets or interactive things you embed in your course do not use Flash. Flash often will not work by default in most browsers and devices, and even Adobe, who makes Flash, is discontinuing it completely. H5P is one alternative free tool for adding some interactivity to your course. In the worst case, you can use a screencast tool such as Screencast-o-Matic or Screencastify, to record a video of the flash animation and share it via Youtube or another video server.