Description of problem:
Most commons that I have ran into are "var" and "end". 

rule test
 when
   Event( start:date)
   Event( end:date) // This actually still works ok
 then
   end.anyFunction(); // Does not reopen in guided rule editor
end

Additional info:
In 5.x we added the dollar sign on front of each variable to avoid this. end became $end. This might be a lot of work with what we have now. Another option is to block the user from adding reserved words.