As a "web designer/publisher" (sorta), I get really mad when other web designers/publishers produce shoddy work.  One big example of this is with links.  Arguably the most important part of the internet is its ability to link different objects and ideas together.  That, along with the whole "information" part, make up the "information superhighway" (wow that's an outdated term).  So messing up links is a pretty big deal.  Here are the three biggest mistakes I see on a regular basis: 

1.  Omitting the "http://" (a.k.a. "scheme").  For example: 

<a href="http://ddhr.orgwww.google.com">www.google.com</a>

What happens if my link doesn't include the scheme?  It doesn't friggin work. 

2.  Including an extra "http://".  This is most likely the result of using an automatic link generator.  For example: 

<a href="http://http://www.google.com">www.google.com</a>

What happens if my link has an extra scheme?  If doesn't friggin work. 

3.  Extra spaces.  This is most likely the result of double-clicking on a word or series of words and then using an automatic link generator.  Double-clicking (at least in Windows) always selects the space after the word.  Double-clicking on the "Dave" in "Dave is a jerk" will select "Dave ".  So a link will look like

the <a href="http://www.google.com/">link is </a>here
or
the<a href="http://www.google.com/"> link is</a> here

This guy does this constantly.  What happens if the markup has extra spaces?  Nothing at all.  But it looks terrible, especially when links are underlined. #technology