avrdude: verification error, first mismatch at byte 0x001a 0x64 != 0xcf

Using Arduino IDE 1.x
1

my arduino nano suddenly have this problem
avrdude: verification error, first mismatch at byte 0x001a
0x64 != 0xcf
avrdude: verification error; content mismatch
avrdude: verification error; content mismatch
what can i do to fix it?