After adding a keyword through the web interface, the interface is in "edit existing" mode, with the newly added keyword in the edit box. It would be more convenient if it allowed adding further keywords. 

Moreover, "Edit Existing" looks very similar to "Create New", so one could easily think that you still are in create mode, and enter new keywords. At least, that's what happened to me. I entered 3 keywords in a row without noticing that the second and third were just edits to the first entry...