People who flipped between WBZ-TV and NECN for coverage of Boston’s Fourth of July celebration might have been confused Wednesday night. Starting around 9:35, NECN showed a silent Hatch Shell stage and people leaving the Esplanade to escape a passing thunderstorm. Meanwhile, on the CBS affiliate, the Boston Pops were performing, right on schedule. On NECN, the Pops’ storied performance of the 1812 Overture was scrapped — but on WBZ it played on as usual. Huh? The explanation: WBZ was airing footage of the dress rehearsal recorded a day earlier. The broadcast used about 45 minutes of recorded video while the crews and attendees waited out the storm. “You have to do so for inclement weather,” David Mugar, the businessman who has produced the show for 39 years and its CBS broadcast for the past 10, said Thursday. “It is common for any live event.” (WBZ ran the words “previously recorded” on the rehearsal video.) Last year’s Fourth of July broadcast on WBZ drew criticism after it was revealed that images of historic Boston landmarks such as Quincy Market and Fenway Park had been inserted into video of the fireworks.