When Texas’ early voting process started last week, some people finished filling out their ballot only to see that their choices had been changed — either switched from one party to another, or erased completely. This seemed like a bug at best, or deliberate election tampering at worst. But state officials have declared that the machines are functioning correctly; people are just using them wrong. And they’ve decried a “disturbing trend” of misinformation and exaggeration, which they worry might dampen enthusiasm during an unusually competitive election year. At a time when election boards are facing very real threats of foreign interference, though, a voting machine that doesn’t accurately record votes has also raised major, understandable concerns — even if it’s just the result of a bad interface.

This seeming “vote-flipping” occurs with the Hart InterCivic eSlate, an electronic voting machine that’s been in use for nearly two decades. The device shows its age: it looks like an outsized PalmPilot, and voters make their selections with a clickwheel and button at the bottom. Around one-third of Texas counties — 82 of 254 — rely on eSlate machines, and in the vast majority of cases, they accurately record voting results. But if voters touch the interface while the page is still loading, or use the clickwheel and button at the same time, it can change their ballots without the user realizing it — especially if they’ve picked a one-button straight Democratic or Republican ticket, since it takes several seconds for the machine to check the boxes for each race.

James Slattery, senior staff attorney at the Texas Civil Rights Project, tells The Verge that around two dozen straight-ticket voters from 10 counties have called to complain about “flipped” ballots. The voters generally noticed before they’d cast their final ballot, so they could go back and correct the errors. And the number of complaints was minuscule compared to the 55,000 ballots collected on the first day of voting alone. But Slattery emphasizes that even a small number of miscast votes can undermine faith in the whole process. “The fact is, it’s deeply unnerving to voters when they intend to select one thing, and then they get to the end and see that the machine appears to have done the exact opposite,” he says.

The Secretary of State’s office has admonished voters to fill out their ballots slowly, emphasizing that “the voting machines are not malfunctioning” and that the issue was purely “user error.” As writers and security experts like Matt Blaze have pointed out, however, users shouldn’t be expected to adapt to a bad interface — especially since the Secretary of State’s office also noticed the issue during the 2016 election. One county election administrator told ABC News Houston that he’d seen the problem cropping up for at least six years, prompting the Texas Democratic Party to criticize the Republican-led state government for “doing nothing” about the problem.

Secretary of State communications director Sam Taylor has emphasized that Texas officials can’t simply update the software, and that it would cost $50 million to upgrade the machines themselves in just three counties. The Hart eSlate machines were last officially certified in 2009, and they’re not the newest Hart voting product on the market. So while the software has been updated over the years, moving from Windows 2000 to Windows 7, there’s not much incentive to fix a relatively small problem — especially since the machines can’t be connected to the internet for these updates.

It’s less clear why users don’t seem to have been explicitly warned about this interface quirk, although as the Secretary of State’s office points out, they did place signs telling voters to check their ballots carefully. You can also practice using the machines with an online simulation.

Electronic voting machines in general have far larger problems. They can contain serious security holes, and if a purely electronic system like the Hart eSlate does malfunction, there’s no paper record to check the votes against. Some Texas counties have been working to improve the system — Austin is set to start using a custom system called Star-Vote in 2020. But that’s exactly what makes problems like this “user error” so serious. If an electronic ballot doesn’t accurately reflect users’ choices, there’s no way to detect how they really wanted to vote — and while some people may have caught the mistakes before casting a ballot, others might not even realize they accidentally switched their votes.

Texas is set to abolish straight-ticket voting by 2020, so this particular error might not be an issue in future years. But Slattery says it’s representative of much larger issues with Texas’s voting system — like the fact that you can’t register to vote online, among myriad other restrictions that stand between Texas citizens and the ballot box. “The fact that you have machines that date to the beginning of this century, using interfaces that people have long since gotten out of the habit of using, is just one sign that Texas elections really need to be brought into the 21st century,” he says.