I don't see this happening. If someone gets marked, then quits 3 ticks later, then logs back in 30 seconds after that...99% of the time it's shady.

I agree you shouldn't punish someone for the first time, unless there's something that makes it obvious it was done in bad faith. (Like dude taunts the assassin after logging back in.)

But if it happens more than once? Odds are pretty small that it was accidental.