It can happen due to a notification coming in from a payment gateway for a subscription that no longer exists or usually from a plan being edited and its parent changed after people have already subscribed. For example the following.
Plan 1
- Plan 1a
Now with the above a user subscribes to Plan 1a. Next the below is done changing the plan structure.
Plan 1
Plan 1a
However their subscription will have been created to keep note of its parent, which is Plan 1 and isn't the case anymore.
Note this can cause errors to log, but it won't actually hurt anything and often can just be ignored.