"This combination of Metformin and alcohol can also lead to some potentially severe consequences, one of which is a resulting possible side effect of lactic acidosis. This can be fatal if not treated in time. This condition can occur when the blood does not have enough oxygen, which is required to transport the glucose throughout the body. The glucose then transforms into lactic acid. Although this is a naturally produced chemical agent within the body, increased levels can result in potentially fatal toxic effects.
This resulting lactic acidosis and Metformin side effects with alcohol consumption will first appear in the form of breathing difficulties, fluctuations in heartbeat, nausea and vomiting."
That's the dire warning over metformin, which is why the no alcohol rule applies. But the warning signs come first, and I had a look online myself initially, and couldn't find any actual cases of it happening in reality.