RIVIERA BEACH, Fla. — The nuns at Zina Rodriguez’s Catholic school in the Bronx thwacked her knuckles to punish sloppy handwriting, so she was shocked when her mail-in ballot in Florida was rejected because her signature did not match the one on record with elections officials.

Ms. Rodriguez, a registered Democrat, found the rejection notice in her mailbox at 7 p.m. the night before the Nov. 6 election, two hours after the deadline for appeal had passed. When she protested at the Palm Beach County Board of Elections the next morning, she learned that the culprit was a driver’s license signature, hastily squiggled on an electronic signature pad two years earlier.

“There were 13 amendments on that ballot. The only reason I chose to write in was because I wanted time to research all the questions. I was fulfilling my responsibility,” said Ms. Rodriguez, 47, a behavioral health care consultant from Lake Worth, Fla. “All of that got thrown away because I wanted to get out of the D.M.V. office as fast as I can. It is incredibly upsetting.”

The issue of faulty signatures, especially on mail-in ballots, has emerged as a central point of contention in the county-by-county recounts taking place in Florida, with lawsuits spinning off the 2018 election like tornadoes off a hurricane.