As a secret shopper, I have been asked to do some things that seemed a bit, well, odd to me. Those were pretty tame though: things such as buying mismatched shoes or asking silly questions. I have never flooded a basement or crashed a car as part of a mystery shop, but Colin Shaw has.
Shaw owns a mystery shopping service in the UK, and his clients include banks, car companies, computer brands, and insurers. Although much of his work consists of tasks that most mystery shoppers would find familiar (e.g., interactions with customer service staff at banks, phone calls to call centers, etc.) some of his work with insurers involves extreme measures such as buying a car, crashing it, then going through the claims process to evaluate the service provided to customers of the insurer.
You can read more about Colin Shaw and his mystery shopping experiences in this story by the BBC.