While visiting the Stack Overflow help center, I found a typo/error. Where I can report the mistake?
In first block it shows +10 in thumbnail and +5 in description.
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You can post about these issues here, or on the Meta for the site in question. In this case, that would be Meta Stack Overflow.
It would likely be best to check a couple other sites. If they have the same error, feel free to post it here! Just be sure to check first to ensure the error hasn't already been reported.
If the error is only on one site, or you don't want to take the time to check other sites, post on that site's Meta. The team sees posts from all Meta sites, so they'll see it either way. However, most questions that affect just one site will be closed if posted here, on the main Meta.