Yup, it happens — fairly often, apparently. Reader and West Oaklander Rachel Reynolds wrote in saying that she received an absentee ballot in the mail a few weeks ago — which she promptly/understandably threw away, given that she hadn't requested an absentee ballot and figured it was just yet another piece of BS election mail. But when she went online to check where her polling place was, she found that, as it turns out, she — and anyone living in a precinct with fewer than 250 registered voters, which is the cutoff that justifies having a polling place — was automatically registered absentee, and that the ballot she'd received was intended to be her official means of voting.
The whole process "made me feel intensely, intensely disenchanted," Reynolds said, adding that a few of her friends and coworkers were also in he same boat. According to Alameda County Registrar of Voters Dave MacDonald, this isn't entirely uncommon. (And, it's worth noting, unfortunately, the precincts with sparse voter numbers are significantly more likely to be populated by minorities and lower-income folks.)
If you're concerned that this case applies to you — meaning you haven't gotten polling place information and/or have checked online and found that your precinct doesn't have a polling place— you have two options for getting your vote counted: You can either go to any polling place and fill out a provisional ballot, or you can go to the Alameda County Registrar of Voters any day up to or including election day, and staffers will find your ballot. If you have strong feelings about local races, the first option may be more risky, insofar that the people working at the polling place may not know exactly where you fall with regards to district boundaries for races like, for example, city council or BART board. But either way, VOTE.