The mistake by former Collingwood and North Melbourne AFL star Rocca - who had blasted one punt 60 yards earlier in the game - gave the Broncos possession on the 35-yard line with the score tied at 21-21.
Gun quarterback Peyton Manning engineered a quick touchdown for Denver who then ran away with the game, outscoring the Redskins 31-0 in the final quarter.
Manning threw for 354 yards and four touchdowns and the Denver defence came up with a season-high five turnovers - more than enough to overcome three interceptions and a lost fumble by the Broncos quarterback.
"That stuff happens in football," Manning said.
"We haven't had a ton of that, but the fact we were able to overcome it was critical."
The Broncos' rally spoiled the return to Denver of former Broncos and current Washington coach Mike Shanahan.
He coached the Broncos from 1995-2008 and won two Super Bowls with the club before he was sacked at the end of the 2008 campaign.
In other action, Matthew Stafford's one-yard touchdown plunge with 12 seconds left lifted the Detroit Lions to a 31-30 victory over the Dallas Cowboys.
The Lions trailed by six points with 1:02 remaining, launching the winning drive at their own 20-yard-line.
They covered 80 yards in less than a minute, and a clutch 22-yard pass from Stafford to Calvin Johnson set Stafford up for his scoring leap.
David Akers added the extra point that sealed it for Detroit, who improved to 5-3.
"It was a great sequence by them," Dallas coach Jason Garrett said. "We didn't do what we needed to do to win the game."
Detroit's victory kept them in second place in the NFC North behind Green Bay, who beat Minnesota 44-31 in a division clash.
Dallas fell to 4-4, but they remained atop the NFC East thanks to Philadelphia's 15-7 loss to the visiting New York Giants in a division matchup.
Josh Brown kicked a career-high five field goals and the Giants shut down an Eagles offense in which rookie Matt Barkley was pressed into quarterback duty after Michael Vick aggravated a hamstring injury in the first half.
The Kansas City Chiefs remained the only unbeaten team in the league, turning back the determined Cleveland Browns for a 23-17 triumph.
The Chiefs gained 281 yards in the first half, but only 50 after the interval as they struggled to keep the Browns at bay.
"It was like two different games," Chiefs head coach Andy Reid said. "It was awfully close."
It wasn't close at all in London, where the San Francisco 49ers trounced the winless Jacksonville Jaguars 42-10 at Wembley Stadium.