Yee, on the stage where you go around a big square loop pushing blocks into corruption. It's honestly longer to grab the key and go back around than it is to just use the key the intended way because you already need to grab the disk right next to the door, but if you do decide to double back around you just have that extra key for a bit.
On the upside, you don't carry it on to the next stage! But it might work just as well to only put one key in the loop and replace that first locked door with another corruption you can fill, effectively just making it a "shortcut" sort of one-way gate. Either way, it doesn't have a huge impact on the end result, it's just something I noticed while going through it.