ATTiny flash verification error

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I've had this same error several times, the first thing to check would be the clock rate of your programmer, in order to properly program your target the SPI rate of the programmer has to be 1/4 or less the rate of the target chips clock speed, or this will occur. Programming succeeds but it's like a baud rate mismatch on a serial port because there's no verification during writing, it just slams the bits blindly, anything over 1/4 of the targets SPI clock will result in bad bits.

Set the programmer to use the slowest possible clock for programming and see what that gets you. Fast programming is only important in production applications generally, so using the lowest possible clock by default is always a good idea.