I've been an overseer at election booths several times. I kept a good look at the transparent ballot box, along with complete strangers. In the end, once, I may even have inadvertently recognized my envelope and bulletin during the opening and counting - because we where very very few people to choose that candidate in my polling station :D And I was lucky enough to follow the president of the polling station to the town hall where armies of public servant where doing the addition, and I checked the CSV on next Monday, etc, etc...

In theory, it's much easier to fake a number stored electronically in a computer than a large number of paper ballots.