Here's an example of how misleading news gets made. A story about the Taliban getting their hands on batteries, for their American made Stinger anti-aircraft missiles, has been circulating around South Asia. The rumor is that "Pakistani government officials" have given Stinger batteries to the Taliban, so they can revive their 1980s vintage Stinger missiles, and proceed to shoot down lots of U.S. aircraft. The reality is a bit different.
Some 2,000 Stinger missiles were given, in the 1980s, to Afghans fighting Russian invaders. Most of the missiles were not used, and most of those subsequently disappeared, instead of being returned to American control. The missile batteries are dead by now, and the rocket propellant has gone bad as well. Moreover, you cannot just get some geek to cobble together new batteries. The "Stinger battery" also contains cooling elements that make the missile seeker work (by allowing it to pick up the hot exhaust of a jet engine.) The rocket motor is only good for 15 years (after that it will start to degrade and give erratic performance.) Replacing the rocket motor is even more difficult that trying to rig replacement batteries. Other components are also expected to become unreliable after two decades, not to mention the damage done by rough treatment the missiles may have received by the tribesmen holding then. In other words, those 1980s era Stingers are useless unless you replace most of the components. It's unlikely that Pakistan has any capability to refurbish Stinger missiles.
The real danger is from Russian SA-7 portable anti-aircraft missiles. Not as capable as the Stinger, there are still lots of SA-7s available with good batteries. Several have been fired in Iraq recently, although without bringing down anything. In Afghanistan, there are lots of small aircraft and helicopters flying around that are very vulnerable to an old-tech missile like the SA-7. During the 1980s, the Afghans got their hands on lots of SA-7s, fired over 500 of them, and brought down 47 aircraft and helicopters, and damaged 18 others. During the Vietnam war, 528 SA-7s were fired, bringing down 45 aircraft and helicopters, and damaging six others.
These missiles were originally intended for use against jet fighters operating low over the battlefield, but the reality turned out to be different. The most likely targets encountered were helicopters, or propeller driven transports. These aircraft proved to be just the sort of thing twenty pound missiles, with 2-3 pound warheads, could destroy. Against jet fighters with powerful engines, the missiles caused some damage to the tailpipe, but usually failed to bring down the jet. This was first noted during the 1973 Arab-Israeli war, where the Egyptians fired hundreds of SA-7s at Israeli A-4 light bombers. Most of the A-4s, with their 11,187 pounds of thrust engines, survived the encounter. Larger jets, like the F-4 and it's 17,000 pound thrust engines, were even more difficult to bring down. Smaller commercial jets, like the 737 or DC-9 (each using two 14,000 pounds of thrust engines) have proved vulnerable. But a 757 has much larger engines with 43,000 pounds of thrust, and the 747 is 63,000. Moreover, the rear end of jet engines are built to take a lot of punishment from all that hot exhaust spewing out. Put a bird into the front of the engine and you can do some real damage. But these missiles home in on heat, and all of that is at the rear end of the engine. Since the 1970s, about 40 commercial aircraft have been brought down by SA-7s, killing over 500 people.
Russia no longer makes the SA-7, but does manufacture more modern versions, closer to the Stinger in capabilities. Egypt and Pakistan do still make versions of the SA-7. There are still tens of thousands of recently manufactured SA-7s out there, as well as many of the more modern versions (like the SA-18). These are the missiles you have to worry about. Many SA-7s have been found in Iraq and Afghanistan, and some SA-18s have shown up in Iraq. We know from experience that for every ten SA-7s you fire, you are likely to bring down a smaller aircraft or helicopter. An SA-18 is about twice as effective. These missiles are designed to be used by untrained troops, and take some rough handling in the field. One thing that discourages their use, aside from the fact that most will not bring something down, is the fact that they do not have a long range (about four kilometers), and leave a distinctive smoke and flame trail that shows nearby troops or police where the missiles were fired from. These angles should not be underestimated, for they appear to be a major impediment to more widespread use of the missiles.
American helicopters and aircraft are equipped with missile detection and protection (lasers or flares) systems. Twenty year old Stingers, even if they were still in working order, would be no more effective than some of the more modern Russian missiles on the black market.