Cathy likes to joke about my conspiracy-mindedness. I like to makeup conspiracy theories as jokes. Here is some fuel on rigged elections. The comments make some valid points about why this would be hard to do in practice such as the injected number of votes not matching the recorded number of voters.
I really like the comment that suggests we get a vote receipt like an atm receipt. When the polls close all votes are published on a website. You could then match your vote receipt to the published vote to see if it differed. If it differed then you could dispute it. This would not allow for changing votes because you are reporting that it is different than the information printed on the vote receipt, not that it is different from your memory.