Sometimes - not often, but often enough to be really annoying - in Author mode I find there is a delay of a second or so between pressing [ENTER] and the content completion context menu appearing.
During this time any keys I press are applied to the main editing screen. For instance, if I type the first two letters of the element that I was wanting to select from the menu that I am anticipating will appear, I end up with those letters actually typed into my document, even - I think - in an element-only context in the XML document (and I have the option set to prevent typing where characters can't be entered). Weirder still is if I use the cursor keys during this period: I find that I move through my document and that when the menu does finally open I get the menu appropriate to the place where I activated it (i.e. before the cursor keys were pressed) rather than the place where the cursor has moved to; this then allows me to put elements in places where they would not be allowed by the schema.
I realise that the obvious workaround is for me to wait for the menu to appear, and that's what I do (when I remember!). But I do wonder whether there is some bug in the input-handling after the [ENTER] key has been pressed, which should be changed so that keys pressed during this interval are either stored and applied to the menu or are ignored.