The following command will recreate the bug.
nmap google-gruyere.appspot.com --script http-grep --script-args 'http-grep.builtins ={"discover"}, http-grep.url="/432894489680891189865518341201629552984/snippets.gtl?uid=zl3"'
the host is up recreate it anytime.

When will it happen and why
There is a bug if doubled is greater than 9. doubled is not gsubbed correctly after line 160
doubled = string.gsub(doubled,'(%d)(%d)',function(a,b)return a+b end)
for example if doubled = 14, after the line above double will be 5.0.0
and it is added to s2 so it will raise the error as below:

NSE: http-grep against google-gruyere.appspot.com (216.58.193.84:443) threw an error!
/home/zheli/nmap_no_interning/nmap/scripts/http-grep.nse:189: attempt to perform arithmetic on a string value (local 'doubled')

the line number is different because I print some debug info in my local file.