As reported in https://bugzilla.wikimedia.org/show_bug.cgi?id=35592 :
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When I review code, the checkbox is set to "reviewed" by default. This is
dangerous (you view and leave the code diff, and the code gets "ok" reviewed).
It should be unchecked by default .
If possible, the standard setting ("reviewed=ok" or "reviewed=not ok" by
default) could be perhaps become a user preferences option in the "Gerrit
reviewer preferences", if you understand, what I mean.
A green check mark appears when returning from a quick view to a changeset, even when I did not any code review.
The standard default (status of the "reviewed" box when visiting a changeset) could be a setting in the user preferences, default = (reviewed=unchecked).