I oversaw a site migration recently. A large part of the legacy site's URL structure changed and we installed Jumplinks to manage all the redirects.
Since the launch I've been monitoring 404s in Google Analytics and we appear to getting quite a few (around 40 per week). What's odd is that the affected URLs all have jumplinks in places, and when I test them the redirects work perfectly fine.
I can't see why Analytics would be picking these up but I'm quite concerned- if anyone as any insight it would be greatly appreciated.