Hidden character Shift-Arrow Up-Backspace
I can't reproduce this either. However, I suggest that you hold down Command and then press Delete, as an alternative to your method. This will delete everything on the line that's left of the cursor, and will hopefully solve your problem. (You can hold down Option to delete single words. Experiment with holding down the Option or Command key, and pressing either Delete or the arrow keys. If you hold down Shift with either Option or Command and then press the arrow keys, you will be able to select text word by word, or line by line.)