Circular references is a computational problem, which cannot be resolved by a computer because it has no logical beginning and end, for example: Enable question Q1 if question Q2 is enabled, and enable question Q2 if question Q1 is enabled.
In this example each of the questions Q1 and Q2 depends on each other, and there is no way to establish whether both should be asked or none at all.
Survey Solutions is capable of determining this kind of design defects and many other, less obvious ones, collectively known as "circular references". For example, question Q1 may depend on question Q2, which depends on question Q3 and so on, with say, Q100 depending on Q1. The chain of references can be arbitrarily long, but as long as there is a circular reference it will be detected and reported.
Another situation is where the question depends on itself, such as with the filtering condition: while the filtering expression may refer to other questions in the questionnaire, it may not refer to the question itself.
Circular references are reported as critical errors during the questionnaire compilation stage. As all other compilation errors they must be addressed before other testing tools can be used to test the questionnaire.
Note that you may, (and in fact expected!) to refer to the value of the question in it's validation expressions. You can do this by referring to the question by it's name or with a convenient self alias.