Mixing up users in version history

I have a weird issue with a customer that I can't explain and can't reproduce. 

Context

I've used an out-of-the-box task list where people can track issues against a project. I've customized the list by adding several text and choice fields to the list and a lookup to a custom project list. Another thing I've done is, I've hidden the people picker and replaced it with a custom implementation to limit the number of people that are returned.

It's good to know that I've deployed this list using remote provisioning to several 100's webs in a site collection and that I'm using a custom content type based on the task content type.

One of the fields I've added is a notes field, on which I've set the option to add comments to the existing text using version history.

The issue

Several users are complaining that it looks like they have added notes to a list item while they can pinpoint the note being added by a co-worker on a different day. Both the user and the timing sees to be incorrect. It shows in the item itself, as well as in the version history. As my customer is using this list in projects they are using it to prove who said what and when. 

I've checked if it's concurrency, which it can't seem to be as SharePoint handles concurrency issues. There's no other customization on this list, no external updates to this list through api's etc. I'm assuming but fairly certain people aren't sharing laptops or credentials.

Question

If somebody is familiair with this issue I would love to know. Steps to reproduce or figure out this issue would be very helpful as well. I'm also very open to the possibility that it's an user error, however I can't figure out what a user might do to create such an issue.