Sometimes lines aren't broken after failing to fill the previous line perfectly

[wasamasa]

See the "No editor comes close." sentence in the middle of the picture. I can understand that without hyphenation the justification algorithm can fail to fill lines perfectly and sometimes leaves a word sticking out the line. What I don't understand is why it can happen that the next word isn't put on a new line. Is this some special case TeX deals with on a case-by-case basis?

[Fuco1]

Well TeX would try REAAAAAAAAALY hard to hyphenate and not leave an orphan, or just stretch the hell out of everything. It has an almost infinite penalty to have just a single word on a newline. Now probably shifting comes close would induce too much error on the previous line... but frankly I guess I should just ignore it on the last line as a special rule and always break two words.

This might also be a bug as it seems to only happen on the last line?