apache - mod_rewrite to redirect raw percent sign in URL instead of Bad Request 400 -

i have bunch of urls following:

http://domain.com/is-20%-into-401k-enough/ 

i aware that, standards, malformed uris therefore going url directly result in bad request server.

here few questions:

1. i thought browsers automatically encode server sees %25 instead of %? why doesn't work in case?
2. can use mod_rewrite redirect malformed uri correct 1 (/is-20%25-into-401k-enough/)?

here i'm @ think bad request getting priority because rewritecond seems looking %25 in url:

rewritecond %{request_uri} \% rewriterule . $0 [b] 

thanks in advance.