What to do about an 'illegal Choice' error in views?
'An illegal choice has been detected. Please contact the site
  administrator. '


This type of issue generally arises when, for example, you have 2-3 select list, select list 'second' is dependent on 'first' select list. Now if you get empty result (say) in your first select list, second select list will not get populated correctly and throw this type of exceptions.

This is just an example so that you might get an idea, the sequence of selecting values is not right.

Try the whole procedure again, and this message will get disappeared.