A number of voters have reported problems with a certain type of voting machine in Texas.

Some voters complained that machines flipped their straight-ticket selections to the other party in key races during early voting, especially the much-watched Senate battle between Republican incumbent Ted Cruz and Democrat Beto O'Rourke.

The Secretary of State’s office issued an advisory about the problem.

"We have heard from a number of people voting on Hart eSlate machines that when they voted straight ticket, it appeared to them that the machine had changed one or more of their selections to a candidate from a different party. This can be caused by the voter taking keyboard actions before a page has fully appeared on the eSlate, thereby de-selecting the pre-filled selection of that party’s candidate…” said Keith Ingram, Director of Elections.

This specific machine uses a wheel and an enter button to maneuver the ballot and enter selections. The advisory goes on to say they should not be used simultaneously.

"The Hart eSlate machines are not malfunctioning, the problems being reported are a result of user error — usually voters hitting a button or using the selection wheel before the screen is finished rendering," said Sam Taylor, spokesman for the office of Secretary of State Rolando Pablos, who was appointed by Republican Gov. Greg Abbott.

You can click here to read the entire advisory.

According to the Dallas Morning News, Tarrant County and 81 other counties use those machines. Dallas, Collin, and Denton counties do not.

Early voting in Texas began Monday and has featured strong turnout and long lines. It runs through Nov. 2, ahead of Election Day on Nov. 6.

Many Hart eSlate machines used in Texas don't provide receipts or other forms of paper trail to voters, but those casting ballots do see a screen that shows their choices before final submission — and can go back and make changes. Similar machines are used in parts of Indiana, Kentucky, Pennsylvania, Tennessee and Virginia, according to Verified Voting, a nonprofit group focused on ensuring the accuracy of elections.

The machine's manufacturer, Hart InterCivic, attributed the Texas issues to 16-year-old technology.

"The same story has happened in multiple elections," Steven Sockwell, the company's vice president of marketing, said Friday. "There was no flipping then and there's not any now."

Instead, Sockwell said, what typically happens in cases where someone believes his or her vote has been changed is a voter will select a straight-party ticket, then unintentionally change votes in individual races without realizing it.

Still, in a statement to supporters, Cruz cited "multiple reports" of race selections changing and added "once you select the Republican party ticket, please be patient and do not select 'next' until the ballot has populated all of the selections."

The Texas Democratic Party called the issue "a malfunction," said it was causing Democrats to inadvertently vote for Cruz and accused the secretary of state's office of not doing enough to warn voters of potential issues.

Party chairman Gilberto Hinojosa said in a statement that "Texas' Republican government blamed voters and did nothing." He called for a statewide public service announcement to warn voters, training for poll workers on the issue and removal of "all malfunctioning machines."

Taylor said Friday that his office "has already trained election officials across the state" while also instructing "election administrators to post additional signage in multiple languages" and requiring county officials to keep "a detailed, meticulous log of any malfunctioning machines, and remove any machines that are malfunctioning."

Taylor also said his office "has no legal authority whatsoever to force any" voting machine vendors "to make upgrades if their voting systems are otherwise in compliance with federal and state law," and that Hart eSlate's system was certified in 2009. He said counties are responsible for purchasing their own new voting equipment.

"We will continue to educate Texas voters using existing resources," Taylor said, "and urge all Texans casting a ballot to take their time, slow down, and carefully review their ballot before casting one."

NBC 5 spoke to the Tarrant County Elections administrator Heider Garcia. He explained that everyone should make sure to check their ballot on the final screen before casting it.

“I think that is the most important part. Go through the ballot, if you waited in line 20 minute 30 minutes, you might as well take an extra 30 seconds. Make sure that that summary page lists everything that you intend to vote for, and then hit the red cast button to make sure it is recorded,” said Garcia.