Its the kind of gaffe that drives aviation enthusiasts into a spittle-flinging rage.

In the highly complex world of warplane watching there is no greater sin than mixing up your different variants.

For those not well versed, it can get confusing with the tuck of tailpipe here - or the extension of electronic warfare sensor there.

Even the experts have trouble keeping up.

The Canadian Armed Forces posted a photo of three Australian fighter jets to its official Facebook account on Tuesday shortly after the Liberal government confirmed it intended to buy 18 used FA-18 "Classic" Hornets as a stopgap ahead of a full competition to replace the air forces aging CF-18s.

The caption read: "We've announced our intention to pursue the purchase of F-18 aircraft from Australia. These aircraft will supplement our current fleet until the future replacement fleet is fully operational."

Problem is: It was wrong plane.

The jets in the photo are the more modern variant of the F-18, known as the Super Hornet.

That was the plane the Liberal government had intended buy before a trade dispute erupted with Boeing, the U.S. manufacturer of all Hornets.

A spokesman for National Defence acknowledged the mistake.

"In our haste following yesterday's important announcement augmenting the fleet, we posted the wrong photo," said Dan Le Bouthillier in an email.

The Canadian Armed Forces posted a photo of F-18s to its Facebook page to tout the government's decision to buy 18 of the fighter jets from Australia. Problem is, the photo depicted U.S. Super Hornets - the plane the government decided not to buy. (Facebook)

He said there was great public interest and a lot of comments on the acquisition.

"We regret the error and will be issuing a new social media post with a correct photo."

Decision in Conservatives' sights

Meanwhile, the Opposition Conservatives continued to dog the Liberals over the decision to buy the used jets on social media and on Parliament Hill.

Foreign affairs critic Erin O'Toole, an air force veteran, tweeted that "40 years ago, [former prime minister] Pierre Trudeau modernized our Air Force with the F-18. Today his son is replacing our used F-18s with... used F-18s. #MerryChristmas RCAF!"

He asked followers to retweet a photo entitled "Justin's used jets. Condition as-is. Share if you think our troops deserve better."

The Liberals have attempted to cast the blame for the slow progress of the fighter replacement program on the Conservatives, saying the former government of Stephen Harper had more than enough opportunity to buy new jets.

On Tuesday, the Trudeau government announced the beginning of the competition to replace the CF-18s, which were all purchased in the early to mid-1980s.

Procurement Services Minister Carla Qualtrough laid out a roadmap that will see a formal tender call in 2019, followed by a contract-signing in 2022 and delivery of new warplanes by 2025.

In the meantime, Canada will augment its current fleet with the Australian jets, which are all of the same vintage as the CF-18s.

"So we're getting a rusty bucket of bolts for our airmen and women to fly," said Conservative MP and former cabinet minister Tony Clement. "This is bad for the Royal Canadian Air Force. It's bad for the Canadian taxpayer."

The decision to go with used fighters brought with it echoes of the controversial late-1990s acquisition of used British submarines, one of which caught fire on its maiden voyage to Canada, killing one sailor.

The country's top military commander, Gen. Jonathan Vance, said that's not an accurate comparison.

"We wouldn't have brought this forward as an option and wouldn't be proceeding on the purchase of this interim fleet unless we knew we could fly it and be confident in it, quite a different situation than the submarines," Vance said on Tuesday.

"Remember as well we are experts at running CF-18s. We know this fleet extremely well."