The management shelves of your local business school library are filled with examples of businesses who believed that success in one market was a sure fire guarantee of success in another. They were wrong and they usually paid a very high price for their error!
Sometimes the new venture is a completely random departure - there is no rational linkage between the exisiting business and the new opportunity. It is the product of an excess of enthusiasm or a lack of critical judgement - or more likely a fatal combination of both. Sometimes, the step is so small as to appear almost inconsequential - an obvious, logical progression. An example might be moving from selling a product to renting the same product instead, or selling what appears to be an obviously complementary product. Both are fraught with peril - a step into completely uncharted waters.
This is one of those situations when asking someone in the team to play "devil's advocate" - is not just desirable but an absolute necessity. All the assumptions need to be thought through, laid out and then challenged and tested. The evidence, if there is any, needs to be reviewed and re-evaluated - is the analysis accurate, is there an alternative explanation for the observed behaviour, how significant and/or representative was the sample, do we have the right skills and the right people. Most importantly, do we have enough cash to fund this new venture?
The danger is greatest, when the person championing business diversification is the boss. MDs take note. This is where you can demonstrate genuine leadership. Develop a rigorous process to test and challenge new business ideas. Then, make it absolutely clear that your own ideas will be subjected to the same level of scrutiny as anyone else's.
It is far better to trash an idea in the board room than let it escape into the market - where the financial cost will no doubt be far higher but the damage to reputation may be incalculable.