Sharp undercut
A groove-like depression melted into the base material along the weld toe caused by inappropriate welding parameters, acting as a severe stress concentration point under dynamic loading.
A sharp undercut is a notch root that acts like a crack. It usually occurs due to too much energy or wrong welding parameters, when the melt pushes up the base material too much and leaves a sharp edge. This thing is dangerous because it acts as a predetermined breaking point – under load it tends to tear further there. You definitely have to rework the notch, either by welding over it or by grinding it out and creating smooth transitions. When welding, make sure to have a calm weld seam and a clean transition to the base material, then you avoid the problem right from the start.