Ever since the newest version of PyDev came out (v2.7.3), I've had an annoying problem with the red underline for errors. Most of the time when PyDev reports an error in my code, and I correct it, the red X icon in the gutter goes away, but the red underline remains.
This only seems to happen when I save the file shortly after correcting the error. If I wait for a parse to occur automatically, before I save, both error indicators go away. But if the bug does trigger and I make another change and resave the file, the bugged underlines remain.
The only way I've found to force a bugged line to fix itself is by highlighting the line in some way. Selecting the line, or placing the cursor on on a variable to make instances of that variable on (or around) the bugged line become highlighted, removes the underline.
I'm using Pydev for Eclipse 2.7.3, with Eclipse Indigo SR2, on OSX Mountain Lion. I don't recall this ever happening before I upgraded to 2.7.3.
P.S. Warning underlines do not appear to be bugged in this way.
Oh, and editing bugged lines sometimes partially fixes the underline. For instance, on a line that is almost completely underlined, if I type a space into the middle of a variable/keyword/string on that line, then only the red underline that's underneath that variable/keyword/string will go away. The rest remains.