Copied directly from the post on SRK:

I don't think this will help anyone, but.. I found something interesting about input shortcuts. Well, not as much a shortcut, but the way DP's get interpreted. I found that (unlike CvS2) you CAN'T do:
DF, F,D,DF+P for a DP. It won't come out, you'll just get the punch. At first I thought that Capcom did this because they wanted to be lenient and let ppl get super instead of DP when they did: D,DF,F,D,DF+P (ie, pressing the button one direction too early), but that doesn't make any sense because that input ALREADY is a shortcut for QCFx2 so I can't think of any reason to NOT let DP come out for DF,F,D,DF+P. Interestingly enough, you can't come out of UF either Try it. Jump up rotate through UP/FORWARD to FORWARD and hold it, then finish the DP motion when you land... it won't come out. I tried it with Ryu, Ken, Elena and Makoto but I'm assuming it's something universal. I can't say for 100% that the restriction is for the forward diagonals only. It might apply to any direction if you manage to go to forward without going through neutral, but I can't test that. If the same rule applies the 'official' input for a DP in 3S should change to N,F,D,DF.

I still tested the DP thing with other characters because some other input related things are not. Like HCF and HCB shortcuts. Necro's spin top thing can be done by inputting DB,D,DF,F+P which means you can do it from crouch block, a nifty little plus IMO. Similarly, Makoto can do Karakusa from DF, but Ryu can NOT do mule kick from DB. I find that last bit to be a little disrespectful to Ryu