Hybrid Flu Virus: What Could Possibly Go Wrong?

Some strains of the flu virus, like the H5N1 bird flu virus, are highly lethal but aren't easily transmitted from humans to humans, whereas others are very contagious but mild. What would happen if the two are mixed to create a new, hybrid virus that is both highly lethal and contagious?
Apparently, a team of Chinese scientists haven't seen the Hollywood movies about such dangerous experimentations and went ahead to create such a new virus:

They combined H5N1 avian influenza, which is highly lethal but doesn’t transmit easily between people, with the highly contagious H1N1 swine flu strain responsible for infecting tens of millions of people in 2009.
The new hybrid virus passed easily between guinea pigs, which are used to study how flu infects mammals. Molecular changes in the virus may provide clues of what to look for in circulating H5N1 strains, perhaps allowing scientists to anticipate when viruses will more easily infect humans.
“Mammalian-transmissible H5N1 viruses can be generated in nature,” said virologist Chen Hualan at China’s Harbin Veterinary Research Institute, who led the research team. “High attention should be paid during routine influenza surveillance to monitor such high risk H5N1 hybrid viruses in nature.”

What could possible go wrong? Brandon Keim of Wired has the story: Here.