I’ve noticed a couple of mojoPortal sites throwing errors when I click image buttons in the administration pages.  For example, when editing page settings, clicking the little spanner icon to edit the settings of a feature instance would crash the site. This only started since I upgraded to IE10 (on Windows 7). 

This is typical of what I was seeing in the system log:

2013-04-17 19:15:57,147 ERROR 192.168.54.5 - en-GB - /Admin/PageLayout.aspx?pageid=282 - mojoPortal.Web.Global -  Referrer(ttp://www.mysite.co.uk/Admin/PageLayout.aspx?pageid=282) useragent Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.1; WOW64; Trident/6.0; BOIE9;ENUS)
System.FormatException: Input string was not in a correct format.
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)
   at System.Web.UI.WebControls.ImageButton.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.WebControls.ImageButton.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection)
   at System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad)
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

Joe Audette pointed out a possible solution in this thread so I installed .Net 4.5 on the web server and the problem was cured.