Apple’s stock is currently trading around $511, the lowest price the company’s shares have been traded at since February of this year. Why? There are concerns that Apple still has supply chain issues related to the iPhone 5 and the iPad mini. It’s also possible that part of the sell off was due to investors taking capital gains ahead of potential tax hikes.

According to Jefferies analyst Peter Misek, Apple is still facing “assembly execution issues [that] are taking longer to resolve than expected.” In addition, the firms that supply parts for Apple’s iPhone 5 have reportedly seen “larger order cuts” during the past two days. That’s alarming, because Apple’s iPhone 5 just launched in 33 new countries today and earlier reports suggested Apple’s supply problems had been ironed out.

As a result of the supply issues, Misek now estimates that Apple will sell 48 million iPhone units this quarter, down from his original estimate of 52 million units.