Use of the "Bypass" button in some (all?) Calf plugins, produces a little "click" or brief scratchy noise.
There perhaps should be a common way (i.e. common library code) for Calf plugins to "smooth out" this audio glitch occurring on use of the respective Bypass buttons; anyway, that's merely an implementation detail.
The plugins I am currently using are MultibandCompressor, Eq5, BassEnhancer, Emphasis and Limiter. Each of these plugins appears to have this audio glitch bug.
I am using Debian sid, and have jack configured so there are no xruns.