When you delete an object type <objectname> from a project you receive the warning:
Deleting this object type will remove all actions, conditions and expressions relating to it, as well as all of its instances. Are you sure you want to delete the object type "<objectname>"? Yes/No
However, when you delete a global variable you receive no similar warning. All of the conditions and expressions related to it are deleted. You can undo if you notice in time, but if you don't you are hosed.
Please consider displaying the same type of warning for global variables.