Not printing on bed. Why?

[thefoxtrot]

I calibrated it and it was able to print the tests that came in the pre-loaded list.

When I tried to download things from Thingiverse such as a Companion Cube from Portal or a COG from Gears of War, the printer nozzle starts from a very obscure height. Not only does it do this for the prints that I tried to tell it to do, but it also did this sometimes for the Shroud Fan and Layer Fan (I simply re-ran it and it was able to get the shroud and layer fan printed without issues.

I tried scaling the small Companion Cube (see attachment), and it resulted in the same nozzle height problem as the other attachment.

What kind of problem is this?
How do I fix it?
Why is that happening?

[KAS]

Sometimes Mattercontrol did this on me a few times. Can you reset the Z though the LCD?