Veniam petimus damusque vicissim - Horace
Translation: "We give this privilege and receive it in turn." Horace
Referenced by Katya
Most common use is given to "pardon" the meaning of forgiveness, and then the phrase means having to understand the faults of others as you want to be pitied their own.
This line is applied, and is particularly applicable, to authors who, as none of their works can attain perfection, should be mutually indulgent.