You're set up on a prime bank. You've seen hoppers along the bank and there are rising trout along the bank. You set up to cast and let it rip. Your loop is tight, you have the distance dialed in and you let your cast go. Your loop unfurls and your foam hopper drops right into...the tall grass right above your target water, or the overhanging willow, or the rootwad in the bank. Wherever it lands it is definitely not in the water where you had intended and definitely not in the water that you were casting to and where that big brown is waiting for a tasty morsel to float by.
What's your natural reaction? Mine, especially for the first several years that I fly fished, was to pull, yank, set, and jerk the fly out of the cover. I'll be honest, my natural reaction still pops it's head up from time to time. What was the result of my angling panic? Usually a broken rod, bent hook, broken or frayed tippet, lost fly or the fly shooting out of the cover and halfway back to me in the middle of a run.
What's a better response? Point your rod tip down and let a downstream facing "bow" form in your fly line. This bow in the line will apply a light, constant pressure to your hung up fly and will, more often than not, result in your fly falling into the water, right off the bank. This is right where you wanted to be in the first place. What won't happen? The fly will generally unsnag itself and will not get hung up, dig it's hook deeper into the shubbery, result in broken tippet or a bent hook.
This reaction comes natural to some and is learned by others. Place me firmly in the learned category. I still fight my natural reaction to yank my fly out of danger and I usually pay the price for it. So, remain patient and you might just be rewarded with a pretty spectacular payoff.