Canon USA issue the following Service Advisory for two of their products :
Except as described below, the following will occur with the affected units listed below:
Linked shooting and wireless flash shooting with radio transmission will not work in some setting modes when 7 or more units are linked. This phenomenon does not occur when 6 or fewer units are linked.
Speedlite 600EX-RT and Speedlite Transmitter ST-E3-RT
Units with a combination of 3rd and 4th digits in the serial number in the ranges shown below are affected :
Speedlite 600EX-RT: [xx01xxxxxx] ~ [xx13xxxxxx]
Speedlite Transmitter ST-E3-RT: [xx01xxxxxx] ~ [xx03xxxxxx]
Note: If the battery compartment of an affected unit contains the marking illustrated below, the product is NOT affected by the phenomenon described above, even if the serial number is within the ranges mentioned above.
To arrange to have an affected unit inspected and serviced for this phenomenon at no cost, please contact Canon using the information below.
This information is for residents of the United States and Puerto Rico only. If you do not reside in the USA or Puerto Rico, please contact the Canon Customer Support Center in your region.
Thank you for using Canon products.
Customer Support Operations
Canon U.S.A., Inc
Contact Information for Inquiries in the United States
Canon Customer Support Center
Phone: 1-800-OK-CANON (1-800-652-2666)