Very frequently I come across professional websites that are missing content, or contain errors in syntax, grammar, and/or accuracy of information provided. Having worked on designing and developing websites, I know how hard it can be to manage large sites and how easy to miss small mistakes. It's certainly presumptuous to assume that because something is off the developers and designers are unskilled and incompetent. But from the consumer standpoint, even small errors stand out, and they deserve the extra time, patience, and energy to avoid them.
I won't name names, but just yesterday I was on the official website of one of the largest electronics companies in the world. The site was a subsite on a new technology, and I happened upon a page where a crucial image was missing. The site is very professional and thorough (you wouldn't expect anything less given the size and status of this company) so it was probably a server issue or a small oversight on someone's part, but for me it underlined the point that we've come to expect such perfection from brand names in particular, that a single error in a site that contains thousands of pages gets registered in memory.
Another example I just came across was a healthcare provider's website. Other websites claim to sell this provider's products in my area, whereas the provider's own website claims they do not service this area. A healthcare facility in my area claims to work with this provider, so my hunch is that the provider's website is inaccurate. This relatively small error could be costing this company potential business.
Small erorrs can affect corporate image, so I don't think enough can be said for paying attention to detail at every level, from the homepage served to millions to esoteric internal pages, the presentation and informational accuracy of which could make or break a potential client's buying decision.