In r54084 I've noticed that text selection by dragging either refuses to work or selects the incorrect text when the source view is scrolled horizontally. This doesn't happen in a build of WebKit from earlier in the week (the source view wraps lines rather than providing for scrolling).
Steps to reproduce:
1) Inspect a page, then switch to the Content view of the main resource.
2) Resize the window slightly to ensure that you have a horizontal scrollbar.
3) Scroll to the right by a small amount.
4) Attempt to select some text in the source using the mouse.
Depending on how far you're scrolled and which text you attempt to select, you'll either end up creating a selection to the right of where you intended, or no selection at all.
Fixed in http://trac.webkit.org/changeset/54133.