Zabbix agent was monitoring it's own log file (at Debug level 3). log item key was used:

log["C:\Program Files\Zabbix\logs\zabbix_agentd.log",,,,,]

Server is configured in time zone UTC +2:00 with automatic DST time change.
On 03:00:00 DST went into effect and time became 04:00:00. When this happened, Zabbix agent started reading and sending the log file from the very beginning.

Windows Server 2008 R2 (64 bit)
Zabbix Agent 2.2.7
Filesystem NTFS