Omg... I have tried to sound the whistle on a major mistake no less than 3 times in the last 7 years and they have all been ignored. I have taken to doing what I used to do with my female friends who had poor taste in men, tell them what is going to happen and let them know the only reason I am doing it is so I can say "told you so" later.
Me: "I don't think this is the right approach, it will cause [problem]"
Them: "No, you don't understand, if we don't do it this way it will cause problems for the client"
[Some time later]
Them: "Mr senior developer, [problem] has happened, you have to help us fix it!"
There's something aggravating about being the go-to person to fix problems and not being trusted enough to have your warnings about upcoming problems he died.