When you enter data in a lookup field that has the automatic resolution and most recently used items functionality enabled, you receive a script error when you tab off the field allowing automatic resolution to choose a most recently used value. When you expand the script error, you may see a detailed script error similiar to the following, referencing MRU fields or InnerHTML:
<CrmScriptErrorReport>
<ReportVersion>1.0</ReportVersion>
<ScriptErrorDetails>
<Message>'this.$E_2.contentWindow.document.body' is null or not an object</Message>
<Line>6</Line>
<URL>/_common/global.ashx?ver=1810813519</URL>
<PageURL>/userdefined/edit.aspx?etc=2&pagemode=iframe&sitemappath=Workplace%7cCustomers%7cnav_search</PageURL>
<CrmScriptErrorReport>
<ReportVersion>1.0</ReportVersion>
<ScriptErrorDetails>
<Message>Unable to set value of the property 'innerHTML': object is null or undefined</Message>
<Line>6</Line>
<URL>/_common/global.ashx?ver=-1915844570</URL>
<PageURL>/userdefined/edit.aspx?
<CrmScriptErrorReport>
<ReportVersion>1.0</ReportVersion>
<ScriptErrorDetails>
<Message>'this.$E_2.contentWindow.document.body' is null or not an object</Message>
<Line>6</Line>
<URL>/_common/global.ashx?ver=1810813519</URL>
<PageURL>/userdefined/edit.aspx?etc=2&pagemode=iframe&sitemappath=Workplace%7cCustomers%7cnav_search</PageURL>
<CrmScriptErrorReport>
<ReportVersion>1.0</ReportVersion>
<ScriptErrorDetails>
<Message>Unable to set value of the property 'innerHTML': object is null or undefined</Message>
<Line>6</Line>
<URL>/_common/global.ashx?ver=-1915844570</URL>
<PageURL>/userdefined/edit.aspx?