Agree with Leafsnail about the "road to hell" saying and have a couple of examples from my view of its meaning. Often, it's good intentioned but with ignorance of the true effects of the actions you're doing.
#1 = prohibition: almost the definition of "well intentioned" action which blew up completely into "hellish" conditions, banning stuff has all sorts of unintended consequences: corruption, crime, gangs, murder, black market, unregulated products, disease epidemics.
#2 = welfare: it has been argued that specifics of welfare allocations has caused more families to break up since a single mother and the father leaving can get more welfare than if they "stick together", as well as fostering dependency issues.
#3 = public advocacy campaigns: often due to human psychology, the effects of a campaign may be totally opposite of the desired effect. I heard about a campaign which basically said "everyone's been stealing X, please don't steal X too" actually caused MORE people to steal 'X', since people seemed to rationalize that "if everyone is doing it, it won't matter if I do too.", they later changed the adverts to show a lone person stealing and some other people looking on disapprovingly, these new adverts had the desired effect. (it was actually a campaign to stop people stealing petrified wood fossils from a national park in Canada).
Interestingly the advert in #3, which backfired, was a lot like the Reagan Era's "just say no" campaign against drugs, which basically told kids "everyone else is doing drugs, but you can 'say no' ".