So I'm a scrub at making gifs/videos, and I think I found something (it doesn't seem very useful, but I might be wrong). I posted the replay onto the 3ds replays under the title "sheikbcncl slide" (sheik needle canceling slide) and the mii "_" (underscore). For filtering, US, Smash, FD, and Sheik. Basically I came across this when practicing breversing needles in training mode. I went to normal smash vs a dummy jigglypuff to record the replay. In the replay, I just tried to do it under the same conditions of my discovery, which was FD and full hand of needles. I first found it on the left side, but after I discovered it works on the right, I went for the right more due to more consistency. I tried to record a previous replay, but I forgot to save it. It left me a bit annoyed so I grenaded a decent amount in this replay.
So essentially what I was doing/ trying to do was breverse at an exact point where it canceled the needles, but I slid onto the stage, much further than a normal waveneedle would let me slide. The very first attempt was successful (somehow), so if you get tired of me failing, you can just rewatch the very beginning over and over. The slide seems to be from me breversing in the air, but since I was still close enough onto the stage, I was able to slide back onto it. I think it is very similar to the post I made earlier about running off BF platform b reverse to get a rather large amount momentum. However, for that one I did not have needles charged, leading me to believe that this is possible to do without a full needle charge (I began speculating after I recorded this because I was not really sure what happened until after I rewatched it). I guess it would work with piercing needles as well. If it does work without charged needles, it probably would have no landing lag, making it potentially useful. (edit: ) I forgot to mention no needles come out despite a full charge.
Anyway, if anyone could point me to a guide for recording/creating gifs, or record it with a capture card and post it (I don't have a capture card), I'd appreciate it. Also if this was already found, go ahead and yell at me for not noticing. I'm going to go back to testing it under new conditions (custom needles/no charge). Right now, I doubt this needs its own thread, so I'm putting it here (should this be a new thread?). Anyway, thanks for reading my essay on a probably not very useful thing. I'll edit in my results of new conditions/discoveries about it when I get it to work. :] This whole thing is probably messy and hard to understand, sorry :[
Edit: piercing needles work
slide appears to have 2 different lengths on stage, and 2 different momentum trajectories off stage, so it really seems like the same kind of slide as the one off the BF platform I was talking about
currently still no success with no charge needles on FD, but I found there is a 3rd slide length for off BF platform, but it appears to only work with charged needles (so basically useless due to landing lag). It lets you slide back just past the platform (left of the red X under platform), or under just the opposite platform (right of red X past that is right of center stage)