What is wrong with this proof?
It is vital that students receive individual feedback specific to their particular misunderstandings.
You can download Geometry checkups: Sample answers which contains two sample proofs for congruence. Both of the sample proofs contain conceptual errors. How would you mark this work and what feedback would you give to the student?
You can also download an annotated version which shows detailed teacher feedback and suggestions for correcting the misunderstandings.
One method to provide individual feedback is to regularly give your students a single problem to complete and submit for correction. Correcting their work and providing timely feedback is a valuable learning experience for your students. Asking for only one proof to be completed is also a more manageable teaching strategy for you.