Sony ericsson once got away with such a trick: Their K800 cellphone was a P.O.S. that broke for no reason whatsoever. The fix? They added a "humidity indicator", which was a piece of paper that turned yellow if a humidity rate of more than 1% was detected.
The manual clearly stated that "your K800 P.O.S. should not be exposed to humidity", but they never clarified what degree of humidity they considered "acceptable".
So, this meant they were legally allowed to void warranties if the humidity indicators were yellow (=exposed to more than 1% humidity, which was apparently their "acceptable limit,").
Most people thought they had exposed the cellphones to a degree of 95% humidity or more, and didn't claim their right, but they had actually exposed it to something like 50%, which is normal, but the indicator turned yellow nevertheless.
So, Sony actually got away with this. And mind you, these where expensive phones, so it would have cost Sony a lot to replace...