You can read the full details here:
Lot Numbers: GD42L and GD52L
Winchester has determined the above lots of 22 Long Rifle rimfire ammunition may contain double powder charges. Ammunition with double powder charges may subject the shooter or bystanders to a risk of serious personal injury and/or death, or cause firearm damage, rendering the firearm inoperable.
DO NOT USE WINCHESTER® M*22™ 22 Long Rifle RIMFIRE AMMUNITION WITH LOT NUMBERS GD42L or GD52L. The ammunition Lot Number is imprinted (stamped without ink) on the left tuck flap of the 500-round carton as indicated here. The 1000-round intermediate carton does not have a Lot Number.
To determine if your ammunition is subject to this notice, review the Symbol Number and Lot Number. If it is Symbol Number S22LRT with a Lot Number containing GD42L or GD52L immediately discontinue use and contact Winchester toll-free at REDACTED or visit http://www.winchester.com/Product-Service/Pages/Contact-Us.aspx for free UPS pick-up of the recalled ammunition.