Running the latest 4.0 nightly (731), I had this problem:
Code like this:
for(int a = 0; a < 256; a++)
I put the caret on the 'a' and did ctrl-r-r - the red box appears around , and a little popup suggestion menu appears containing a few other single-letter variable name suggestions - the top item is selected (let's say it's 't')
I type one character 'x' (which is not in the suggestion list) - the rename happens correctly throughout the function, but the suggestion box does not dismiss
I hit TAB to get out of the rename box - and the variable name changes from 'x' which I just typed, to 't' which was the current selection in the suggestion box.
It seems that the suggestion box does not dismiss until a second character has been typed - I can see why that might be desirable, but the behaviour where pressing 'tab' causes the rename you've just done to change into something else feels completely wrong. *If the selected item in the suggestion box doesn't share a first letter with the single-letter variable which has just been typed, then it doesn't really feel likely that it was the intention of the user to select it.
*I am not a big user of single letter variable names, so I don't know if this problem predates 4.0 or not - playing with it a big more makes me suspect it might - I have a feeling that you're going to tell me that it's by design, and I should get used to pressing ESCAPE to dismiss the pop-up before hitting TAB.
Anyway, perhaps there is some room for improvement here.