I have the following senario:
2 servers part of Log Shipping. Many databases from the Primary Server are
being log-shipped to the stand-by server with no recovery option. All
maintetance plans have been designed with exactly the same options, timeouts
and thresholds. However, two of those dbs on the standby server always get
out-of-sync. By reviewing the logs, I notice that at a point in time, I
start having 2 consequent logs that wait to be restored meaning that the
first of the 2 was not restored when it was supposed to do so. The errors
afterwards is that it cannot restore such old logs.

Another clue that I discovered is that this problem starts at a time that
this database is being heavily updated by clients. All the other hours,
updates are
very rare. Is there any type of update to a db that can cause it to come
out-of-sync ?

Please note that I have tried several different values for the backup times
and out-of-sync thresholds but in vain.

Another question: If you use Log Shipping and at the same time you perform
Transaction Log backups, will this affect the log shipping ? (because you
have intermediate logs that are not being replicated to the standby server?)