In 流石, さす is phoneticized as “sauce”.
But in 注ぐ, そそ is phoneticized as “sauce”.
As a sidenote, this is similar to: https://community.wanikani.com/t/the-same-mnemonic-is-used-for-both-すう-and-す
But in some ways it’s worse, since the problem appears twice in the same word (inconsistencies for both the first and second syllables).
But in some ways it’s not as bad, because I don’t actually get hung up on either of these words (whereas with すう vs す I definitely did).
Thanks - brought this up with the team!
This is one I definitely got wrong a few times because I couldn’t remember which version of the “sauce” mnemonic to use.
So I just made a new one for 注ぐ. You’re pouring some “so-so goo.” It’s a thick sauce that tastes just ok (so-so).
Ironically, I think writing this post stuck both of these kanjis in my brain for long enough that I’ll never get them wrong again, even without any mnemonics. 
My new mnemonic device is… complaining!
It’s crazy how effective it can be. Just the other day I was complaining to my (thoroughly uninterested) wife about having to learn a useless phrase like “Bitter Enemies In The Same Boat”, and now I don’t think I’ll ever be able to forget it.
The content team has reviewed this, and we have now updated the reading mnemonic for 注ぐ.
Thanks! Not usually a fan of mnemonic changes, but at least I know there’s a good reason for this one. Presumably maybe all of the others that I’ve been annoyed by had similarly good reasons that I was unaware of. 