Here are some common CSS mistakes we all make
 

Not Using a Proper CSS Reset. Web browsers are our fickle friends.
Over-Qualifying Selectors.
Not Using Shorthand Properties.
Using 0px instead of 0.
Using Color Names Instead of Hexadecimal.
Redundant Selectors.
Redundant Properties.
Not Providing Fallback Fonts.

Yes! But it's all very common mistakes and everyone has their specific way of writing codes whether you are a beginner.