Re: Impetus to check
On legality there would be two main issues (disclaimer IANAL).
Firstly did you release the false information? If the answer is yes then that is market manipulation if you (or others connected to you) intend to trade on it and very very illegal.
The second is are you acting on material non public information? IE do you know anything that an average guy on the street couldn't easily find out? If the answer is yes then don't trade.
So lets say you didn't release the data, you have seen the release, realised its fake and seen the beginning trend in sell orders through purely public information. Lets also assume you have a spread bet account for a highly leveraged trade. You could probably make such a trade and were you to be investigated for possible insider trading or market manipulation then prove yourself innocent.
Would you want to take that risk (along with the risk of actually being right that the market will move down, what time to buy, what time to sell, whether to hedge against the underlying markets and whether you wanted to reverse the position to catch the bounce)?