[At the 1958 conclave]:

Questions began to arise whether the smoke was white or gray. To quell such doubts Monsignor Santaro, Secretary of the Conclave of Cardinals, informed the press that the smoke, indeed had been white and that a new pope had been elected. The waiting continued. By evening, Vatican Radio announced that the results remained uncertain.

According to Lawrence Elliot,

In fact, the result was sometimes confusing. On Sunday, the smoke seemed white enough and the Vatican Radio and at least one news agency flashed word that a pope had been chosen. But as the minutes passed without light or any sign of activity in the Hall of Benedictions on the second floor of St. Peter’s and the facade of white-curtained windows remained dark and mute, the people in the piazza and the millions around the world who also waited realized that the wait
still went on.

The partly white, partly dark smoke confused even Vatican Radio announcers.
They had to apologize frequently for their error. The column of smoke which rose from the chimney of the Sistine Chapel was first whitish, then definitely white, and only later definitely black.

In his book, “Above All a Shepherd: Pope John XXIII,” Ugo Groppi wrote an account of an eyewitness who was just feet away from the Sistine Chapel chimney:

An editor of the “Osservatore Romano” who was in the office of the Secretary of State saw the smoke from a distance of several yards and immediately dashed off to the newspaper in order to prepare an ‘extra.’ But as he entered the editorial room he heard the radio announcing that the result of the balloting was negative; a black smoke signal had followed the first white one. The same mistake was repeated on Sunday afternoon. On Monday morning learned discussions were taking place all over Rome, especially among so-called experts, on the nature and
the color of the smoke signal.