It happens mostly when I'm trying to highlight a collocation. I've tried different browsers - the same problem.

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I used to experience the problem from time to time. I made sure that I save my words first, in other words, get rid of the blue words, and then go back in and save phrases. This has eliminated the problem. We are working on a variety of major changes which should eliminate this problem. In the meantime I suggest you slow down when saving phrases, and in particular save phrases separately from saving words. Avoid having blue words in the phrases you are saving, at least for now.