經而不驗 - To go through without examing.
驗而不經 - to examine without going through.
The first case describes people who have gone through a situation but did not and do not learn from it.
The second case describes people who try to make up what may be the result of a situation and draw conclusion without really going through such a situation.
To make it clearer, the first is to try to establish a rule merely from observed results. The second is to try to establish a theory without practical testing and believe it is true.
At FSRC, students are discouraged to teach before having adequate practical work. This is because FSRC only produces experienced teachers.