QualiTest has recently produced a report and an infographic about the typology and sources of the most common bugs found in web sites. The company has used for this the bug summaries from hundreds of web site testing projects performed recently.
This infographic details the type of bugs where it is not a surprise to see that functional bugs are the most detected defects with 34% of the cases. You will also find in this report information the test cycle in which bugs are detected, the severity of the bugs, their distribution by web browser or mobile OS.
You will find more information about this infographic on http://www.qualitestgroup.com/blog/performance-testing/common-website-bugs/