The removed prefix always ends with a slash, meaning the matching occurs against a string which never has a leading slash. Therefore, a Pattern with ^/ never matches in per-directory context.
Make that "never unless you're getting stuck with severely malformed URLs" ;) The writers of the Apache docs forgot the part about how it's impossible to make things foolproof because fools are so ### ingenious.
Tangential but not irrelevant: Do you have any information on where these // are coming from? Is it in your power to do anything about it? They seem to have hit on the only possible permutation of / and . that isn't auto-repaired in transit.