That is most likely just things poking at your site, not something I would worry about. There was once a security issue in ASP.NET where they could figure out the machine key by the timing of certain errors, when I see these in my logs I'm pretty sure it is bots testing to see if your site has the patches or not. There was a similar way to exploit the same thing using viewstate errors and occasionally I see lots and lots of viewstate errors in rapid succession from the same ip address. When I see lots and lots of errors in a row from the same ip address I usually ban the ip address. But with the script error you posted I usually only see one or two in a row so I don't ban those.

Be careful also about banning based on viewstate errors as there are innocent reasons that error can occur if the user has a slow connection and clicks buttons before the page is fully loaded for example can cause a viewstate error, but when you see dozens of them in row with fractions of a second in their timing thats when I become convinced it is an attack. In both cases the issue was already fixed in ASP.NET so there is no danger of a successful attack.

Best,

Joe