Craig Buckler on Opera 10’s weird user agent string (my emphasis):
Opera is one of the oldest browsers and is the first to approach a double-digit version number. Unfortunately, the Opera developers started to experience strange problems with a selection of sites that use browser sniffing to serve version-specific content and/or scripts. Many of the sniffing scripts simply detected the first digit in the user agent string and boldly assumed they were running on Opera 1 rather than Opera 10. Worse still, many of these sites decided that the browser was unsupported and refused to provide any content.
Which sites? In the comments, David Storey says:
One of the biggest reasons why we did this is because it effects [sic] ASP.net/IIS sites, meaning potentially many thousands of sites are broken by the Opera 10 UA string. These range from AJAX not working, large parts of the code being missing, or being out right [sic] blocked. Major sites like Bank of America were also effected. [sic]