Yesterday, I was going to create a Global Variable but half way through I decided that this did not need to be global and would instead use a local one. So I clicked the cancel button before finishing the creation.
I then went to the rule, created a local variable in there (with a different name than first anticipated), once done, I edited the rule to add the local variable, but when I was asked what rule to use, the name of the global variable (that I had canceled) was in the list of variables to choose.
So after I had finished editing the rule, I went back to rule manager and look at the global variables but nothing in there, so I had to create a global variable with the same intended name I had started first time around, finished the creation of it and then deleted it to get rid of all traces of it elsewhere.
So for some reason when you hit cancel when creating a global variable, the name will remain in the global variables table without actually being there, I did not try to use or anything, so I don't know if would throw errors or not, but I could recreate the bug to make sure it wasn't just me.
Does this bug exists in local variables also, I have no idea but it is possible.