The humble font is not as humble as you might think, especially on a Mac. Fonts are more like plug-ins than you can imagine, and because they are loaded by macOS and the apps running on it, a bad font can have some unexpected consequences.
These consequences may range from the relatively benign, such as some characters being erroneously displayed, to something more annoying, such as apps that refuse to load or functions that do not work as they used to. Incorrect fonts are rarely the cause of problems with the MacOS system, but if you experience problems that don't make sense, you can check out your fonts a good first troubleshooting step to take.
Thankfully, Apple includes a tool for installing, validating, and generally debugging fonts with each Mac. The next time your Mac is abusive and you are out of ideas, validating your fonts is a good troubleshooting step.
How to validate fonts on a Mac
Open the Font Book application to begin. You can find it in the Applications folder. (Open Finder and click Go> Programs to open the folder.) When you are open, select all your fonts by clicking one and pressing Cmd + A.
 Right-click on one of the fonts and click "Confirm Fonts."
When the validation is complete, a new window appears confirming if any of your errors are damaged A green icon indicates that they are good, a yellow icon indicates that there are font alerts, and a red icon indicates that validation failed.
How to remove damaged or duplicate fonts
If any of your fonts are really damaged, select the checkboxes next to them and then click the "Delete checked" button.
The selected characters will be removed from your system. If it is really needed, restart the Mac once it has been written off.