Some minor mistakes won’t invalidate your kit: the sampler being moved a couple inches, getting knocked over but promptly put back, etc. Even forgetting to test under Closed House Conditions or placing your kit directly on the floor won’t invalidate your kit (though it probably will affect your test’s accuracy – refer to the instructions for more info).
Some other mistakes render a kit invalid. If your sampler’s paper backing is punctured, torn, or otherwise compromised, this invalidates a kit. Overexposed kits also result in an error report. In these cases, you will need to start again with a new kit.