Since I am extremely familiar with my local cemetery of over 4,000 burials, I will tell you about three common situations which occur in Find-a-grave.
1) Cemetery records list a Patricia Flynn, female, who died in 1912. I pull her obituary, and it's Patrick Flynn, male. All of his relatives have him dying in the wrong city in the wrong year. *Cemetery records are wrong.*
2) My grandfather was born before his father died in 1892. He was probably born in 1891 (I have two sources for his birth that substantiate that). His tombstone says he was born in 1892, because he really had no idea, so my grandmother used that year. *His tombstone is wrong.* (and if he had a death certificate, it would presumably be wrong too, but no one issued one for him).
3) Our city bought a cemetery management program that requires you to enter a birth month and year and death month and year, even if one wasn't recorded in the original records. Approximately 1/6 of everyone in the records is randomly assigned to being born on the first day of January. Unless I can find independent confirmation for their birth date, I leave that out of Find-a-grave, but others add that in as though it's a legitimate date. Find-a-grave is wrong because the city record is wrong. By using this ridiculous software, the city has created even more errors in the records.
4) Lots of Find-a-grave entries use a burial date instead of the actual date of death. Sometimes they let you know about that; frequently they don't.
If I have evidence, I submit corrections to Find-a-grave, but that doesn't mean they'll ever be changed or updated. Find-a-grave is a tool that can lead one to good research, and it can be especially useful at illuminating possible family connections, but overall, since it is user-created and there's no ability to add sources, it isn't an extremely reliable source. So why bother checking for discrepancies between Find-a-grave and Wikitree? If I've researched someone, added sources, and noted discrepancies in the biography, then why should I worry about discrepancies between Find-a-grave and my Wikitree profiles showing up in error reports? Life is too short. I'm glad that you found actual errors using the data checking, but I looked at my 11 errors, and none of them were actual errors. Either I have evidence that contradicts Find-a-grave or I am still working to resolve discrepancies between our dates.