Just for the records, I now know how I created this junk line:
I had a stack of three items: three identical Pauwalu Degikup Bowl. I used the split (shift+click) option because I wanted to sell them one by one. The first split single-item I put in the backpack.
I splitted the remaining two stacked items, and was moving the one item which was at my cursor directly into the sell window (instead of putting it into my backpack), and got an error message that splitting failed.
VoilĂ*, I had such a junk line again. Both items from the second splitting function were lost.