Leak Behind Residential Microwave

A customer called and said they had water leaking into their kitchen. Troubleshooting revealed the water only leaked out of the wall when the upstairs toilet was flushed. We opened up the kitchen wall in various places looking for the leak. It finally seemed that the leak was coming from behind the wall-hung microwave oven. We removed the oven and found that a hold had been drilled into the toilet drain line when the bracket for the microwave had been installed, over a year previously. (There is no 'after' photo because I forgot to take one.)