You're familiar with the double slit experiment:

http://en.wikipedia.org/wiki/Double_slit_experiment

Adding a detector at one slit causes the wave interference pattern to be replaced by the particle double band.

Now imagine that the target screen has two slits in it, and yet another target screen behind that.

What do we see on the second target? Probably a wave interference pattern.

Now add a detector to one of the initial slits (not in the first target screen). What happens? Probably a wave interference pattern.