Problem uploading sketches

I've been doing all sorts with my Uno over the last week, but after making a few small tweaks to my latest sketch this evening, I couldn't get it to upload. I get the following error:

avrdude: verification error, first mismatch at byte 0x0000 0x0c != 0x62 avrdude: verification error; content mismatch

It won't even let me upload the standard 'blink' sketch.

I've disconnected everything from the board, tried a new USB cable in a new port, tried resetting the board immediately before uploading. Always the same error.

When I push the reset button, led 13 flashes then stays on permanently, sometimes flickering a little.

When I upload, the TX and RX lights flash, but after the error it's back to led 13 staying on.

Googling indicates either a fried board or the need to replace the bootloader.

Any advice?

Edit: it's perhaps also worth noting my MacBook has had a kernel panic and shut itself down three times today (an otherwise incredibly rare even) while doing stuff with the Uno.

Hello ArduinJoe,
Kindly add the details of the tweaks. Are you using a dedicated UNO board or breadboard setup with UNO bootloader?

Thanks for the reply.

It’s a dedicated Uno R3 board - not an offical Ardiuno, one of the cheaper ones from eBay.

The only change to my code between it working and not working was to add a couple of lines to print a line of text to the LCD display:

lcd.print("====================");

I had two temperature sensors connected, the LCD display, and an LED from digital output 2 - all of which had been functioning correctly for some time before this happened.

Check this topic http://www.nerdkits.com/forum/thread/1281/. It may be a COM port issue/