Electronic – arduino – Using a breadboard power supply along with USB to Serial chip

One of them is there to prevent damage that could occur if the AVR has RxD programmed as an output, pins on both devices could be damaged if that happened as AVR pins can source and sink quite a lot of current. I don't think that the other resistor is necessary.

I think you're killing the tantalum capacitor across the input. In fact, it looks like it's bulged up in your photo.

Although the capacitor may be rated at 16V (I don't actually see a rating anywhere, but I'm inferring it from the size and input voltage spec) seasoned engineers will never use tantalum capacitors in this sort of situation. The input current needs to be limited by several ohms and/or the rated voltage should be something like three times the operating voltage. Your wall wart can probably supply several amperes.

Years ago, this information was anecdotal, but today you can find it buried deep within the manuals for such parts. They bury it, because it means you very seldom should use tantalum capacitors, and almost never across power supply rails unless the current is limited to much less than an ampere.

Many have been burned by this (perhaps literally). Often failures take place long after product has been shipped, and can cause localized charring, meaning (if your standards are high, as in aerospace or medical) the entire board must be scrapped.