Issue with attaching grades directly to Gradebook (1 & 2) from both the "Tests & Quizzes" tool and the "Discussion & Private Messages" tool. Gradebooks multiply (usually by 10) random students' grades in random assessments. Current issue workaround is to detach and reattach the assessment to and from Gradebook (1 & 2).
How to fix.
Fortunately, a second bug (or perhaps oversight) is that in the original Gradebook, you can still edit a grade even if it came from a tool (GB2 locks you out of editing grades sent from other tools). So, just change all the numbers back to their original values (8000 was originally 80, 8.0x10 to the 15 was original 80, etc).