This might seem obvious, but sometimes the thrill of finding a long-lost relation can cloud our judgement. You must always double-check all your finds, and question the sources.
I think I will share a few of my own error experiences that happened just these past few days, one from badly transcribed records and one from an error in an online family tree.
Census records (or any hand-written records) are notorious for having mistakes in them, usually spelling ones. I was looking for a marriage registration on Ancestry.ca and found a pretty bad transcription:
There were 6 names listed on this document, and half of them were transcribed wrongly. I looked at the scanned original, and I can see that the document itself was correct (but admittedly difficult to read). Since the search function uses the transcribed information, it's no wonder I could never find anything. Searching by Soundex codes can help eliminate all the little spelling mistakes.
Published online family trees are getting to be very common and can be an amazing source of information. But don't assume other people are infallible either. That doesn't mean you have to chase down every resource that someone else may have used (though that's not a bad idea if you have the time). Perhaps make a note in your records that certain ancestor connections are unproven.
My other example comes from such an online tree. I was/am looking for more on Oscar Vinette and found someone who has him in their tree. Their Oscar is married the same woman as my Oscar is, and I was happy to see they had 6 or 7 generations recorded of Oscar's ancestors.
But as I tried to double-check some of the information, I found that this tree has the wrong Oscar. All the records I can find have their Oscar with 2 marriages, and the 3rd marriage (that connects him to me) is just not correct. There just happens to be two Oscar Vinettes. I'm positive of it.
Yes, I was disappointed. But I was also happy with myself for being diligent in my research. Solving puzzles is fun, even if you don't get the results you hope for.