What happened

On Thursday, shares of aerospace giant Boeing (NYSE:BA) gained 2% on news that European anti-monopoly regulators might potentially prevent it from spending $3.8 billion to acquire an 80% stake in Brazilian aircraft builder Embraer Brazilian Aviation (NYSE:ERJ), a company currently worth only $1.1 billion.

Today, Boeing stock is heading in the other direction: down 6.1% as of 1:45 p.m. EDT. And Embraer shares are down 11.6%. Is this just a continuing reaction to Thursday's story?

So what

Yes and no. On the one hand, yes, today's news suggests there's a heightened chance that Boeing's acquisition of most of Embraer will not actually happen. On the other hand, today it's not the EU that's the main threat to the deal, but Boeing and Embraer themselves.

As Reuters reports today, the two companies are currently at loggerheads over whether various conditions for Boeing's acquisition have been met.

As initially agreed, the two companies were supposed to finalize their deal by today. But they apparently haven't quite worked out how to set up and fund a new joint venture containing the 80% stake in Embraer's commercial aircraft division that Boeing will be acquiring. And they haven't agreed to extend the deadline for working out these and other details.

Now what

Despite the precipitous drop in Embraer's share price (down 66% over the past 12 months) since this deal was announced, Boeing is said to still be committed to the deal, although Reuters was unable to get either company to confirm this. But judging from investors' reaction to Thursday's news that the EU could nix the deal, they don't seem so sure that the acquisition would be such a great idea. And today, both stocks are going down on rumors that the deal could fall apart.

This doesn't make much sense, if investors are hoping that it might get canceled. Perhaps investors' real worry today, then, is that Boeing and Embraer will find some way to work out their differences, and that Boeing will end up vastly overpaying for Embraer after all.