Electromagnetic Interference (EMI) can occur between your tablet and other devices. Devices could include cell phones or keyboards. It doesn't even really have to be a device- sometimes ordinary non electronic objects have magnets inside of them that can interfere with a tablet.
EMI can make the pen have jerky movements - making the pen unresponsive for a few tenths of a second and then the pointer will catch up.
Sometimes the EMI takes the form of a region that warps straight lines. Below is an example of such a warping caused by a magnet. This this case it was a magnet attacked to a stand underneath the tablet. (See this reddit thread)
EMI interference can also present as a region where pen seems unable to make a mark. Notice that some warping is also present. In this case, the causes was that the tablet was resting upon a laptop keyboard. Note that there is also some warping of the pen position. (See this reddit thread)
Sometimes it is the tablet that is interfering with another device.
Examples:
A wireless mouse might have trouble working near some tablets.
A tablet might interfere with your WiFi network connection
Weird things can happen! I have a keyboard, that when I unplug its USB-C cable will act like a pen and control the position of the pointer on one of my tablets!
There are computer desks that are made of metal and deliberately magnetized. These can interfere with your drawing tablet. See:
Some iPad Cases have magnets in them and they can interfere with the proper functioning of an Apple Pencil.
/r/iPad - Some magnetic cases may interfere with the Apple Pencil Pro 2024/06/05
