If you are planning to do an In-Place code upgrade (AX2012 to AX2012 R2) and you are on CU6 there is a high chance you will get this little problem.
When you start the client and select the in-place code upgrade you get this error:
"Method SysCheckListItem.getMenuItemType must be overridden."
The solution is simply to just add one single method to one class.
In the SysChecklistItem_FixAxIDs add the method getMenuItemType and make it return the MenuItemType::Display enum value.
Compile the class and try run the checklist again. This is already documented on the download page for the update, but easy to miss. I'll admit I missed it the first time. If you searched for the error, hopefully you came here and you are now already back on track. :-)