From Canon USA :
Thank you for using Canon products.
We have determined that some EF50mm f/1.4 USM lenses exhibit a focus operation-related malfunction. The details of the phenomenon and Canon’s service policy are described below.
We would like to offer our sincere apologies to users who have been inconvenienced by this issue. Going forward, we will spare no effort in our quality management to make sure our customers can use our products with confidence.
While in AF or MF mode, when the focus position is moved from the infinity end to the close-up end, the focus operation stops near the area shown in the dotted circle below.
Possibly Affected Products
If the first four digits (from the left) in the serial number on the lens body (see image below) are “4918”, “5018”, “5118”, “5218”, “5318”, or “5418” then your lens may possibly be affected.
Canon EF 50mm f 1.4 USM Serial Number Identification
Potentially affected products will be inspected and if necessary repaired free of charge. If you own one of the potentially affected products please contact our Customer Support Center.
When preparing your lens for inspection please be sure to attach the lens cap and the dust cap to the lens.
If you have not already done so, please register your Canon Product. By registering, we will be able to notify you via email about future announcements.
This information is for residents of the United States and its five territories only. If you do not reside in the USA or its five territories, please contact the Canon Customer Support Center in your region.
Customer Support Operations
Canon U.S.A., Inc
Contact Information for Inquiries
Canon Customer Support Center