Project 3: verification error

After pressing the upload button, I get this:

avrdude: verification error, first mismatch at byte 0x0002 ** 0x61 != 0x5d** avrdude: verification error; content mismatch

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If I try uploading other sketch, I don't get any error.

Update: only if I upload previous sketches (i.e. Spaceship project), I don't get any error.

What should I do?

I use Ubuntu 14.10