From the server's perspective: I'll never pick up the check accidentally. If I grab the receipt with your tip, it's to send a clear message. Maybe I need your table because I can't make additional tips if you don't leave and it's been a long time since you finished dessert. Maybe I've had cash walk away in the past. Maybe I'm about to be done with my shift and I want to make sure that I receive the tips I earned. If that's the case, I probably owe you a heads-up as to why I'm snagging the check or a well-timed question about whether or not you need change.
From the diner's perspective: This one feels about as kosher as a server who asks if I'm done with my food while reaching for my plate. I don't want to feel rushed and I don't want my tip to be the fastest thing you've grabbed all night — that just leaves a sour taste in my mouth. If you've picked up the tip, it also suggests that I'm not getting any more water or coffee refills, even if that's not the case.
The verdict: While not entirely out of line, this move seems to be poor form. A server can be in a hurry to settle up the bill or run a card, but the meal's final moments shouldn't feel hurried. What do you think? Is it ever OK for a server to pick up the tip before you've left?