Although the secret ballot isn't a palpable concern to me, I guess that's just because I've not had to concern myself with that. Other than ensuring that each voter appears/votes only once, there's been no record in areas that I've voted that could track my actions.

But, some historical examples come to mind from 6th grade civics class -- it may be apocraphal! Operatives of political parties would gather up folks on a wagon and bring them to the polling place. The promise of a shot of whiskey or a couple bucks "motivated" their choices. That would certainly be a lot more effective if you could provide proof, such as a ballot receipt, of exactly who you voted for.