Spelling mistakes make software projects and web apps look unprofessional. When you're reviewing a project, make sure to check not just the main interface elements, but also the error messages.
The screen grab above from eBay demonstrates the issue; anyone for 'posatge'?
Spelling mistakes in these areas often get missed during testing; unless your process is especially rigorous, it's easy to miss some areas of the interface. Mistakes happen, but it's worth keeping spelling in mind when you're going through the review process.