There are so many dirty tricks you can do with electronic devices! The most common I see is the screen calibration trick. If you fail to calibrate it, certain parts of the screen will be off slightly when you touch the screen. So your button press will actually select the candidate above or below your intended selection.
Elections should be a manual process with the ballot box always in view and witnesses allow to stand nearby and watch the vote count at the end of the day. This way everyone can witness the actual count for each precinct. Even if they lie when reporting the precinct numbers to election HQ, it would be caught.
Vote fraud is an inherent feature of any voting system, which is why the paper ballot is the simplest and most secure system yet devised. It's inexpensive, provides a permanent record for recounts, and most important it is massively decentralized.
The latter is key as it means that while it is possible to tamper with an individual ballot box (due to local corruption) it is logistically very difficult to tamper with thousands of individual ballot boxes without such efforts going noticed.
As an election official, you can see if the calibration is off when you turn on the machine and run the test and verify the initial counts are zero. Last election, I took one of our voting machines out of service with no votes because of the calibration. It was off all day. I use a rubber tipped stylus in the center of the button to check the calibration.
“Elections should be a manual process with the ballot box”
Really. When I was doing elections, we had good ol’ IBM punch cards. If there were challenges or similar you could pull the precints’ boxes and run the batchs again and/or do a manual re-count. With the new methods there’s no end to the fraud that can be done.
We can thank Gore for the mess we’re in and the hanging chads in FL.