These are the top ten accessibility errors as researched by TPGi, a company focusing on accessibility. See the linked article for details on each item, as well as instructions on how to do it correctly.

1. No link text
2. Non-active element in tab order
3. Missing link alt attribute
4. No alt text
5. List not nested correctly
6. Duplicate labels used
7. Positive tabindex value
8. Invalid aria-describedby
9. No label for button element
10. Invalid aria-labelledby

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Generally, I am surprised by these items. I would have expected more complex issues, like "invalid markup" or "markup not semantic" (cf. 5). Also, more than "non-interactive elements can be tabbed to" I would have expected "interactive elements cannot be tabbed to". Or missing focus styles.

On the other hand – all these items are not to hard to fix. 1, 3, 4, 5, 7, 8, 9, 10 should be easy to do right, and 2, 6 should be hard to do wrong.