Contact names got mixed up (mostly XMPP)
Some contacts have got wrong names since a while: Wrong names are attached to XMPP contacts, the profile pic (avatar) of another XMPP account is shown for an XMPP contact or both of it. Because of this I already wrote messages to the wrong people.
How to detect: Compare the nick name of the XMPP contact and the actual jabber-ID on the details page of the Messages app.
It's especially bad if you have an XMPP contact which isn't linked to another contact as one can not change the NickName associated to an XMPP contact in the People app. The nick names have either been set by the XMPP contact themselves or by me using Gajim and then have been automatically synced to my Jolla. The Jolla is the only client showing such weird behaviour, the nicknames in Conversations, Gajim and KDE Telepathy still fit.
The nickname-change also affects XMPP accounts which are linked to other non-XMPP contacts. There the effects aren't that bad as you still see the name of the associated contact, nevertheless the wrong nickname is added to the contact, too.
The third variant of this bug is an XMPP account which shows the correct nick name, but the profile picture of a completely different XMPP contact.
This issue exists at least since 2.0.5.6 but I just found the time to report it.
My XMPP server is using prosody-0.9.12-8.1 if it's of any interest.
If it's possible to anonymize/ pseudonymize ContactDB excerpts I'm willing to provide them.
update: This is still happening in Sailfish OS 2.2.x