Here are some brief (and sometimes cryptic) comments on the SRFI-13
reference implementation.  Some of these are just style recommendations.

Brad Lucier

%string-hash:
By anding with the mask, implementation does no follow specification
unless bound is a power of 2.

string-fold-right:
should be (kons (string-ref s i) v), not (kons v (string-ref s i))

string-unfold-right:
start with chunk of size 40, not of size 2.

string-every, string-any:
I'd prefer to use criterion, not criteria.

string-titlecase, string-titlecase!:
Reverse order in file.

string-index, string-index-right, string-skip, string-skip-right, string-count:
Add internal routines that do the work but no error checking.  Call the
string arg s for consistency.

string-pad-right, string-pad:
Need checking for argument n.  Reverse order in files.

string-[down|up]case[!], string-fold, string-fold-right, string-concatenate:
Define % versions that don't do any argument checking.  Use internally.

string-kmp-partial-search:
Need to check i

string-concatenate/shared, string-concatenate, string-join:
Need to check strings.