As bad as the prior story is, I don't know if intentionally misleading the students is the right way either— what if one had realized the contamination and acting in good faith had cleaned out the bottle? What if they did this afterward and ended up redoing the experiment only to be told they had cheated?

I'm all for exposing students to something unknown, but telling them they're doing X when it's really Y for anything longer than a single lecture ain't it.

You can square that circle by announcing at the beginning of the course that there is going to be some assignment like that, but I'm not telling you which, because the real world doesn't.

I do agree this is a good point; trust is not something that should be simply squandered. Nevertheless, this is still a lesson that needs to be taught and so often students make it to the end without a single teacher that did.

Given that a report is supposed to tell what you did and then your calculations and conclusions, you'd better include something as dramatic as “we washed the equipment after getting the wrong results and detecting contamination”…