This is possible because a link to a normal website was redirected automatically to an executable file (probably from apache), as you can see for skype where the page www.skype.com/go/getskype appears in the first position and if you click Skype will start downloading automatically. You also have the choice of clicking the View as HTML link and just as with other formats, the file information is displayed.
Anyway, this must be a bug. I mean what use is from having the executable files indexed, as in the View as HTML section there is no relevant information. Plus this is a security risk, even a high one. Because sites full of spyware might use this redirect bug to have spyware executables indexed and when the user will click it automatically installing all the malware in the world. I've tested this. For instance if you search for Backup4all (a backup program), beside normal results from clean sites, you'll get either on page one or two (depending on the google server) a link from yaguo.com that is an executable file containing spyware (yes I've downloaded this specifially and monitored it, and it installs spyware). So if a normal user searches to install a clean program it's possible to accidentally end up installing some spyware. This is wrong and should be corrected.
