Many things can cause a saw to go lean:
drop in ambient temp since the last time saw was used
saw tuned to 4 stroke in the shop under no load, then goes lean under load when it gets hot
bad fuel pump diaphragm flappers that can't pump enough fuel to keep up in a long cut but seem fine when blipping the throttle
plugged fuel tank vent
air leaks etc.
So what can you do to ensure none of these problems bite you in the butt?
I constantly check for 4 stroking. I fire the saw up and let it idle (if it won't idle when cold it's not right) for a min or so. Then I'll hold it at WOT for a sec or 2. I'm looking for oil flying off the bar and I'm listening for a good 4 stroke burble. If I don't hear the burble and see the oil I stop and fix the problem or pick up another saw. Before I put the bar in the wood the saw is burbling at WOT. When I put the bar in the wood I want to hear the transition from 4 stroking to the normal 2 stroke whine. If I'm in the middle of a long cut I'll lift up a little to unload the saw. I want to hear the transition back to the burble, then I'll let it go back to eating, kinda like burping the saw. I hold it at WOT all the way through the cut, and I want to hear the burble when the cut is finished. The burble upon exiting the cut does a couple things. The unburned fuel in the cylinder which causes the burble also cools down the cylinder preventing scuffing, scoring and other bad things that happen when a saw gets too hot. The burble also tells me that the fuel system is functioning properly to provide enough fuel to keep that cylinder happy under full load and heat.
I realize that I'm in the minority here holding the saw at WOT for a sec or so before the cut, through the cut, and for a sec or so AFTER the cut. IMO the saw is under less stress burbling at WOT, no load, with the flywheel fan at max rpm blowing cool air over the cylinder than it is loaded in the cut. Most folks are scared the WOT rpms are gonna hurt the saw. I've seen guys back off the throttle near the end of the cut and let the saw lug through the last part of the cut because they didn't want to over rev it. The saws already hot from a long cut and now they are lugging it at part throttle. IMO if the saw has a good solid burble at WOT the rpms aren't going to go high enough to hurt it. I've never seized or burned up a saw or thrown a rod etc. I do know folks that have burned up saws due to being lean. I ask why didn't you check for 4 stroking? Well I blipped it a couple times, thought I heard it 4 stroke, so I figured it was good, didn't want to hold it too long at WOT and have it blow up. Well it blew up anyway didn't it, cuz it was lean and you didn't know it because you were afraid of WOT when you should have been scared of LEAN!
Lean is what kills saws, not 4 stroking at WOT. Remember if she ain't strokin', she ain't right!