SUMMARY
Occasionally, the Summoner's Familiar can appear in a different place on the map than its 3D character model, and all skills associated with it will not work correctly.
WHEN
This bug has been present since the Warlock update.
WHERE
This bug can occur in most dungeons and areas -- in this patch, I've had it occur in Sogun's Lament: Interior and in Ancient Lunar Meadow, but it has also happened in Mushin's Tower, Highland Exile, and Bloodshade Harbor: Command Center.
ADDITIONAL DETAILS
It doesn't seem like there is anything specific that can trigger it, but the bug was reproduced by staff in the previous patch -- it just wasn't fixed. It might seem like skills are possible, but they never actually land, so CCing, grabbing, etc using the Familiar is impossible. If you can get close enough to where the Familiar is shown on the map (rather than where its character model is shown), then it can be Beckoned to fix the issue, but this isn't always possible, so it can be debilitating to the class. I've attached a screenshot of the bug occurring. Note that the location of the Familiar on the map is different than where its character model shows.