Sometimes 'connectivity fix' scripts break things for routing. One case is due to human nature (not tagging a bridge for several reasons )
User 1 (me) adds a pair of trails, but doesn't specifically add a bridge.
User 2 (someone else) does a 'connectivity fixes' run, and ends up connecting 2 trails which aren't really connected.
This then results in a map website creating a route which has a user cycle halfway across a bridge, and then jump 50 feet below to a walking trail because it's a 'shorter route'.
I've since tagged that section of bike trail as a bridge, but have seen other issues where mapping websites generate routes that have users also jumping off bridges.
I think most of it comes down to human nature. You map out a track but forget, or don't realize that you actually crossed a bridge when riding.