So how does one avoid the mistake again? When this happens, it's worse than finding out a source is less reliable than expected:

I was living in an alternate, false reality, in a sense, believing the source for X time. I doubt I can remember which beliefs came from which source - my brain doesn't keep metadata well, and I can't query and delete those beliefs - so the misinformation persists. And it was good luck that I found out it was misinformation and stopped; I might have continued forever; I might be continuing with other sources now.

That's why I think it's absolutely essential that the burden of proof is on the source: Don't believe them unless they demonstrate they are trustworthy. They are guilty until proven innocent. That's how science and the law work, for example. That's the only innoculation against misinformation, imho.