AT90USB646_08 Datasheet, PDF (28/30 Pages) ATMEL Corporation – 8-bit Microcontroller with 64/128K Bytes of ISP Flash and USB Controller

If the CPU core is in sleep and wakes-up from an asynchronous timer interrupt and then go
back in sleep mode again it may wake up several times.
Problem Fix/workaround
A software workaround is to wait with performing the sleep instruction until
TCNT2>OCR2+1.
28 AT90USB64/128
7593GS–AVR–03/08