The current version of Calc uses Ctrl + Shift + C as keyboard shortcut to turn on/off track changes mode.
This is a design flaw in my opinion. As a professional spreadsheet user, I use Ctrl + C (for copy), Ctrl + V (for paste), and Ctrl + Shift + V (for paste special) very often. However, the key "C" and "V" is next to each other. This means that, as a human being, it very likely that when I want to hit Ctrl + Shift + V, I may be wrongly hitting Ctrl + Shift + C, which in turn unexpectedly turn on track-changes mode, and I would not realize that.
I never encounter this issue before, but from one of the major new version I started to unexpectedly turn on track changes on when I am doing quick copy and paste operation, almost everyday. Not only me, but also some of my other colleagues encounter the same issue, while they do not realize that it was track-changes turned on when they see the red square around each cells they edit.
So, please, change the shortcurt to some other one rather than Ctrl + Shift + C.
*** This bug has been marked as a duplicate of bug 130847 ***