I was doing some automated checking of our language packs and I noticed there are quite a few strings that have variable references which don't have the curly brackets in the right place - i.e. $a instead of {$a}.
This was a fresh 2.2 install with various lang packs installed via moodle.org. The attached file contains all the files/strings that have errors (87 strings in total).
The command I used to find them was:
grep -Ri '$a[^-}]\|[^{]$a' *
It might be worth running this on all language packs as I only had 13 packs installed.
Simon