So this didn't happen today, but I wanted to share my first adventure into chip writing, as for me it was a huge accomplishment.
So story starts off like this. I had a project in mind that involved a teensy2.0 microcontroller. I was to use said microcontroller to interface with a Microsoft Sidewinder Force Feedback Joystick (the original one, not 2). This involved programming the MCU to interface with the Male DB15 connector of the Joystick. Well the programming part of things was already done by someone else on github and showed the wiring and parts needed to complete said project. I figured great! I'll just buy the parts I need an I'll be good to go. Well trying to save some money I went onto the fleabay and purchased everything I needed including a Teensy2.0++, I figured I'd get the ++ version so I'd have extra ports if I wanted to expand the project even further.
Everything came in no problem, connected up the chinese Teensy and loaded some blinky examples that where available from the Teensy website. They Worked! I had been familiar with arduino coding and so since I was able to load up teensyduino I could program straight from there just to mess around with the Teensy a bit first. After playing around with the blinky program a few times changing the speed and what have you I noticed it stopped wanting to accept new code. No idea what happened I thought all was lost. I mean afterall it was a chinese knockoff so I figured I got hosed. Well several weeks have passed and I've done research over and over again to try to figure out how I can remedy my situation. The chip still worked as it would turn on and blink, it even showed up as being connected to the programmer. However no matter how many times I tried sending it code and it told me it completed successfully, the chip just kept blinking at the same rate even when I changed the speed. So I was at a loss of what had happened.
So I decided that I'd try programming the chip directly. I researched how to do so and found out I could use my Arduino as an ISP to program the at90USB1286 I just needed a 16hz crystal connected to it and everything wired up correctly to do so. So I went through the process of getting what I needed. Because I didn't have any crystals on hand I decided to get some off fleabay because the ones on amazon were fairly expensive compared to the same kit on ebay. I didn't mind waiting the chinese shipping time. However weeks had passed since I ordered those crystals and I haven't seen them. I tried contacting the seller and got no response back. The item showed up as shipped however they never ended up showing up at my door. Eventually I had to put a dispute with ebay to get my money back. So I tried ordering elsewhere and finally got the package of crystals in.
Two months have passed since I started trying to do this, so now I was finally able to connect everything! So I went and looked up how to use the Arduino as an ISP. As this was my first time attempting to do such a thing. Spent the time connecting my pins correctly connecting up the crystal and trying to re-program the teensy2pp clone. Well I had to program the bootloader, so I tried doing it. However it kept failing. I tried to set the correct fuses however the fuses never showed the correct thing. I kept getting error after error, trying different things over and over again. Looking up the diagram on how to connect everything and I just knew it had to be something stupid. But aha! I somehow missed that I needed to power the teensy with a 5v line, and I thought it was being powered, even tho no lights ever turned on. Rechecked my wiring and sure enough I had wires put in wrong. However there was another thing I discovered. I didn't need crystals, at all, because the teensy has one built in already on the pcb. DOPE!

Connected everything back up and wrote in a command to check the current fuse settings. SUCCESS! It was reading the correct values! So I wrote in the command to change the fuses to the bootloader that I was going to load up to it. (HID Device) Success! Connected it up and it worked!
Then the next part was figuring out the code. Originally it was meant for the Teensy2.0 not the ++ I've never really compiled something before so that was a whole journey in of itself. However it wasn't to bad. Figured out that everything was already set up in the code to allow for the ++ I just had to change a few lines of code and I was able to compile it no problem! I connected up the USB, used a different program to program the Teensy through HID, and as far as I knew it said it succeeded. Fantastic! I was ecstatic. Soldered up the DB15 connector to it correctly connected up the Joystick and I'll be damned the thing worked flawlessly! For something that no longer has any support or drivers for Windows 10, my force feedback joystick now worked perfectly with force feedback effects! I was so glad that I didn't waste my time or money on a teensy that I thought didn't work. For me this was a HUGE success as I had never programmed a chip directly before, never had to compile code before, and have done very little reverse engineering to figure out how things worked so I could get it working again.
There is tons of this story that is missing, as I assure you a lot more happened than I wrote, including figuring out how to get the arduino as an ISP correctly, and seeing the LEDs have a heartbeat when correctly connected, which was cool for me to see alone, and a success. However I wanted to share my success story on making a usb interface for a Joystick that was over 20 years old, and to this day hasn't really been matched.