I found the explanation in the K60 manual:
"
Flash Reads and Ignored Writes
The FTFL module requires only the flash address to execute a flash memory read. MCU
read access is available to all flash blocks.
The MCU must not read from the flash memory while commands are running (as
evidenced by CCIF=0) on that block. Read data cannot be guaranteed from a flash block
while any command is processing within that block. The block arbitration logic detects
any simultaneous access and reports this as a read collision error (see the
FSTAT[RDCOLERR] bit).
"