Brad Hargreaves is something of a startup wunderkind. 

Most recently, Hargreaves founded Common, a co-living company based in New York City. He's better known as the co-founder of General Assembly, a global education platform that teaches technical and business skills. The latter has more than $119 million in venture capital funding, more than 25,000 graduates, and 19 campuses in locations from San Francisco to Hong Kong.

With that success, it's easy to forget that General Assembly, much like all startups, went through growing pains during the early stages.

Back in 2012, just one year after launching, and with 10,000 students enrolled, General Assembly made a dangerous bet on the German market.

The company expanded for all the wrong reasons, Hargreaves admitted during a breakfast panel in New York City on Thursday. Hosted by online lending firm CommonBond, the panel brought together players in the startup ecosystem like Kathryn Minshew, co-founder and CEO of The Muse, and Matthew Lieber, co-founder and CEO of Gimlet Media. Maria Aspan, a senior editor at Inc., moderated the discussion.  

"Most of our cities were very organic," said Hargreaves, insofar as the company saw consumer interest for the courses it planned to offer in those locations. With Berlin, "we got paid a ton of money from Deutsche Telekom to come there," he said. 

"One of the toughest parts of being an entrepreneur is deciding when to be opportunistic, and when to be strategic. If you're always strategic, you will miss opportunities. If you're always opportunistic, you will build a schizophrenic business," Hargreaves said. 

Ultimately, General Assembly pulled the plug on the Berlin campus.

It's not always easy to know when to ditch a startup idea--especially one that sounds brilliant at first.

That's something that Minshew, who launched The Muse, knows just as well as Hargreaves. Her company, a career support site for Millennials, has also seen much success. It currently serves over 4 million job seekers every month, and has raised $12.8 million in VC funding. 

Still, Minshew reflected on the fact that her first business--a travel site that would connect travelers with residents, helping them to discover local haunts--was something of a bust.

"I loved the idea, and I thought it was the coolest thing ever," Minshew said. "It was one of those things where it was really, really exciting, until suddenly it wasn't. Some of the obstacles took away some of our passion for it." 

Minshew knew that it was time to say goodbye to the venture when she lacked the motivation to face those obstacles head on. One New York City regulation, for instance, requires that all tour guides be certified, so the business would need to go up against that law, or certify its potential client base of amateur hobbyists.

"It was a motivation question, it was literally us saying I don't know that we believe that this should exist, or care about this so much to deal with this very de-motivating bit of legalese," Minshew recalled.

The key, she added, is to determine who your naysayers are. If that's your target demographic, you have a problem. If not, it comes down to how well you can build out your product. 

"If the problem's there, I think that you can build a solution," she said.