I see lazy plot devices as anything that is too coincidental. Person One just so happened to be in the 'area' when Person Two was attacked (then they end up running into each other a bunch of times afterwards--really?). Those nicks-of-time rescues. Some random thing distracting a villain right before they are victorious and giving the hero enough time to gain the upper hand.
I'd also like to think happenstance/luck/fortune has a place in sustaining tension, such as bad weather preventing the villain from reaching the heroes or certain people's paths so happening to cross at the right time.
How do you draw the line between a plot device that is too contrived and a plot device that propels the story forward without breaking the suspension of disbelief?