Let's say that Player A is playing at Comp+ REL, and misses their trigger on their Duskmantle Seer. Is this considered a beneficial or detrimental trigger? We all know that Dark Confidant is beneficial, but that's because he is played for his trigger (you wouldn't otherwise be playing a 1B 2/1). However, with the Seer, you're left for a 4/4 Flyer for 2UB - a creature that could and perhaps would be played without his trigger.

I was surprised it wasn't talked about in Toby Elliott's recent “Gatecrash Triggers to Watch Out For” (although since it wasn't listed, I guess it's considered beneficial?). I know that it was one that confused me as soon as I saw it. It's one of the closest I've seen to that thin line that separates the two. I've also heard we should err on the side of beneficial, so is this one of those cases?

A point to notice about missed triggers is that ‘beneficial’ is not mentioned anywhere in the policy. The policy simply states that we only give a Warning if the trigger is generally considred detrimental.

From this angle, Duskmantle Seer's ability is clearly not a drawback to the card - so it isn't a detrimental trigger.

As a general rule, symmetrical effects are not “generally detrimental”. For example, Sulfuric Vortex, which does the same thing to both players, though at different times, does not earn a warning when you miss it, no matter whose turn it is (though it would be wise to investigate when someone misses it only on his or her own turn to rule out a “different” infraction).