The work below is by  technicians who called themselves experts, often with years of experience.  But they were not trained artists.
All of these cases would need removal before correction. 
It's always better to make sure it's  done right the first time. 
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Pure black is almost never an appropriate brow color for caucasian clients.  This brow was applied too black,  too solid and too thickly.  It  would have to be removed before it could be redone correctly. 

The eyeliner was implanted too deeply, causing pigment fanning across the entire lid.  This may not be correctable at all.
A doctor tried to camouflage a breast implant scar with "matching" skin tone pigment that was too light and opaque.  Now the scar actually looks raised (hypertrophic) and more pronounced. 
This browless client went to a nurse with twelve years of experience, and still ended up with brows that are flat, radically asymmetrical, and purple!  Can you see the white in this color?  These brows would turn black if treated with laser and only time would tell if they could be removed after that.
These uneven, overdrawn,  spotty, asymmetrical, purple lips were another nurse technician's idea of "natural." 
Bad Work by Other Technicians
Permanent Makeup
Jeffery Lyle Segal