Hi Pasta, I've many a time had spinach, rocket or other bags of lettuce where there's been a few leaves that are brown (wet, soggy brown) that i just pick out but use the rest of the bag. A thing I have been doing for quite a while is:-
Open up the plastic bags of any salad leaf (like a packet of crisps)
on an iceburg lettuce, I open up the sealed plastic wrapping, trim the 'end of the stalk' which goes brown & keep taking a thin slice off, everytime it goes brown. I place a sheet of kitchen paper towel, on the plastic wrapping & place the lettuce (stalk side up) & put it in the salad drawer - it keeps for ages.
The other thing I do is, when you open a bag of salad leaves (like said above) I then place a paper kitchen towel on either side of the lettuce, (like a sandwich) this absorbs any moisture & the bag lasts again for ages