An update to this:
To replicate the glitch, click on weapon A then immediately go to weapon B. After the crafting menu loads up, weapon B, the weapon you last clicked on would have the frame options of weapon A. Experiment this yourself, I personally couldn't get this to work super consistently, but it's not impossible to do. There may be other ways to do it, but this is what I did to make it work semi-consistently. Some weapons don't gain much from certain frames while others can change significantly (refer to Cheese Forever's video).

I originally saw the glitch from Cheese Forever:    • Rare Glitch Deletes Bosses - Fusion B...  

I'm not 100% sure what causes the glitch and exactly how to replicate it, if I have to guess it's likely due to lag, both in game and internet lag. My internet is not in a stable state all day today and I got error coded 3 times prior to crafting the weapon. It's theoretically possible to mix and match all frames in the crafting station but will be extremely difficult to achieve.

I've also managed to get a Glaive frame on a Fusion Rifle, but failed to craft the weapon, there may be more digging to do. I don't encourage people to replicate this to make broken weapon frames in PvP, no one would be happy to fight against this. This is definitely broken and Bungie has to get on this quickly.