Under Law 19 Scrum there is a table that indicates where a scrum occurs for each occasion and there are entries for PK at goal and FK failures. And that's all I could find until I hi-lited and copied the sectiom title and then pasted it here: TAKING A PENALTY OR FREE-KICK Sanction: SCRUM.
Yes!!!!! There is the mystery sanction! Not visible in my PDF copy but shows up when copied & pasted!! :wtf:
Wow, thanks for spotting that, somebody must have selected the wrong font color, or forgotten to add a line break! In the app it's the same, "Sanction: SCRUM" is not visible in the title above item 20.5. I've just sent World Rugby a link to your post.
That said, I found 3 more slight differences:
1. regarding penalty or free-kick taken at the wrong place:
- in the 2017 law book, law 21.2.c clearly states that only a quickly taken PK/FK taken at the wrong place is to be re-taken by the same team (if it's not quickly taken it's a scrum)
- in the 2018 law 20.2 states that when a penalty or free-kick is taken at the wrong place it must be re-taken, without mentioning that it only applies to PK/FK quickly taken.
2. regarding intent to kick at goal at PK:
- the 2017 Law book is clear that failing to respect the intention to kick at goal (21.5.b) results in a scrum
- the 2018 Law book does not seem to explicitly state the same (8.20, no sanction specified).
I fail to see what other sanction than a scrum could be applied here (unless it's PK for behaving against the spirit of good sportsmanship maybe

).
3. in 2018, it is now made clear that opponents are not allowed to shout during a kick at goal (8.26). Shouting was only explicitly prohibited for conversions
(feel free to correct me if I'm wrong of course!)