• Well-meaning laws requiring baby car seats to be put in the backseat, increase incidence of babies dying of exposure in a hot cars. The likely mechanism is that if the baby is in the backseat it's easier to forget them as one is leaving the car. Note that although this may be an unintended consequence, the unintended consequence may not be greater than the harm to babies from riding in the front seat, those would have to be weighed against one another.
• Increase in incidence of malaria when well-meaning Peace Corps workers helped install new stoves in huts that alleviated smoke inside the huts. The lack of smoke presumably allowed the mosquitoes into the huts or near the huts, thus spreading malaria. I think this was mentioned in the book Guns, Germs and Steel. Traditions sometimes have a sort of collective intelligence that isn't obvious.
• More broadly, banning products or services that are in demand creates black markets, which often come with their own problems.
• Selling products or services for less than their market value can convert cost to other forms such as long lines - the people are then "paying" by standing in line rather than just paying a greater price to begin with.
• Uber Taxi's Gouging with Peak Fares - People seem to often not understand that Uber (company) taxis charging much higher rates on New Years Day in the early AM hours, is not necessarily a bad thing. Shouldn't Uber be "reasonable" and altruistic by keeping the prices the same as off-peak hours instead of gouging at the peak times? If Uber was "altruistic" keeping the prices consistent, there would be the unintended consequence of long and/or unpredictable wait times, leaving many people waiting perhaps until 4AM. Wouldn't you rather know the price up front and decide at that point if it's worth it to pay the peak fare, or if you should find another way home?

A general point: unintended, doesn't mean unforeseen and it doesn't mean that the unintended consequences make the initial action overall a net poor decision. Granted, many of the answers to this question are unintended and unforeseen. But there very well could be foreseen unintended consequences.

For many of the other answers to this question, it's not clear whether or not the the unintended consequence makes the endeavor overall a net poor decision. It could be that though the consequence was unintended (and either foreseen or not foreseen) there are other benefits to the endeavor that ultimately make it a net positive.