This happened to me once after I spilled some tea on my keyboard. I had to replace the keyboard.
One idea: check mouse and keyboard connections.
It might be a faulty keyboard, or faulty mouse, or faulty connection of either keyboard or mouse to the computer. -- (It almost sounds like you're holding down the shift-key between strokes.)
Also, if you're using a usb hub between keyboard and/or mouse and computer, only plug keyboard and mouse directly into the compuuter.
But I don`t think that is the reason there -
It comes both on the touchpad on mac book pro and with the brand new magic mouse
(I also think I once tried the same problem with another computer). No other problems.
Look at the image - maybee that is easier than understanding than my english.
There is the strokes -
Is this happening all the time and with different (all) images? I've not heard of it before, but it is the sort of glitch that might be fixed by resetting Preferences. I think you said you are using a Mac ,so hold down Shift Opt Cmd while starting Photoshop.
If that doesn't fix it, come back.
Is it a desktop or laptop? If the latter, then maybe to do with the touch pad. If the errant line appears straight and parallel from the first, then possibly the Shift key is sticky.
Is it a wireless keyboard? If yes, do you have a plain vanilla keyboard you can try (one that doesn't have features that need a driver to load)
Do you have a tablet? Try disconnecting it, or moving (well) away from computer if wireless)
That's enough to be getting on with.
Thank you for the answer. Great
I hold down Shift Opt Cmd while starting Photoshop - and got the question about resetting - yes
So now it is working.