Could the following error be caused by differentForm.navigate() call combined with the parallel script loading challenges? (This isn't the only error message that I have seen, but the longest one.)
Microsoft Dynamics CRM Error Report Contents<CrmScriptErrorReport><ReportVersion>1.0</ReportVersion><ScriptErrorDetails><Message>Unable to get property 'get_defaultValue' of undefined or null reference</Message><Line>1155</Line><URL>/_common/JsProvider.ashx?IsRefreshForm=true&ids=1545465446-1759665639-1968334880-202661451-1547461133-1992083491-1000289424-159335543-1194605645-1039841082-1314140682-1793205875-1376927455-1030632819-1181113682-232397045-468302605-1405388298-1731109441-2029667886-399776211-330369912-202888856-204060739-1025323794-513446117-1625018438-1460109179-1088147315-706656670-342519603-542820515-1946640899-1603536461-1220618931-511599809-1264077421-1188909594-783705869-374257838-502328314-1391165063-60010994-2050812538-512064351&ver=-1367595599</URL><PageURL>/_forms/read/page.aspx?_gridType=3&etc=3&id=%7b21CF9445-998E-E311-A4B4-6C3BE5A8C0E8%7d&pagemode=iframe&preloadcache=1391711204720&rskey=20891584&theme=Outlook15White</PageURL><Function>anonymous(){var$v_0=this.get_editControlBehavior().get_defaultValue();return!isNaN($v_0)&&!IsNull($v_0)?Mscrm.NumberUtility.addFormatting($v_0,this.get_$0_3().get_precision()):""}</Function><CallStack><Function>anonymous(){var$v_0=this.get_editControlBehavior().get_defaultValue();return!isNaN($v_0)&&!IsNull($v_0)?Mscrm.NumberUtility.addFormatting($v_0,this.get_$0_3().get_precision()):""}</Function></CallStack></ScriptErrorDetails><ClientInformation><BrowserUserAgent>Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; WOW64; Trident/6.0; .NET4.0E; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 2.0.50727; .NET CLR 3.0.30729; InfoPath.3; ASU2JS)</BrowserUserAgent><BrowserLanguage>en-US</BrowserLanguage><SystemLanguage>en-US</SystemLanguage><UserLanguage>en-US</UserLanguage><ScreenResolution>1093x614</ScreenResolution><ClientName>Web</ClientName><ClientTime>2014-02-06T12:26:47</ClientTime></ClientInformation><ServerInformation><OrgLanguage>1033</OrgLanguage><OrgCulture>1033</OrgCulture><UserLanguage>1033</UserLanguage><UserCulture>1033</UserCulture><OrgID>{2F567A75-0703-4F3B-8680-825EEA12F28C}</OrgID><UserID>{AC039637-FC83-462F-B2AA-2F57DFE0E417}</UserID><CRMVersion>6.0.1.462</CRMVersion></ServerInformation></CrmScriptErrorReport>The script resides on the OnLoad of the Opportunity which uses the form matching a value on the record. I've read that there are known errors from JsProvider.ashx, possibly specific to Opportunity. But there is no error when the script doesn't run, so I doubt that it is entity related.
Also, this is testing for upgrade to 2013 and there is no error on the production 2011 instance.