In Mathematica 8 one could use Ctrl+K to complete a word within a Text cell. If there was more than one possible word a menu of words would be provided from which one could be selected. This was quite useful, both for spell checking and just for searching for a word - at least for those who like to write notebooks containing textual discussion.
In Mathematica 9 this still worked but on each occasion an error message would appear that recommended a report to WRI giving the conditions under which the error occurred. I certainly did this a few dozen times hoping there would be a correction in Mathematica 9.0.1. No such luck, but no great harm - just move the Message window out of the way where you won't notice it.
In Mathematica 10 the feature did not work at all. Ctrl+K only produced a small black square at the upper right hand corner of the screen. But heck, that's progress because there was no error message. I did receive an email from support stating that the problem had been fixed in Mathematica 10 and then, upon inquiry, an email stating that is was fixed in Mathematica 10.0.1.
In Mathematica 10.0.1 there was definite progress. Ctrl+K now quite reliably crashes Mathematica. No black square, no error message. Oh, and as an added feature, Spell Checking, Alt+;, also crashes Mathematica.
So let's not be negative. In the next Mathematica it might crash the computer, or maybe take down the Cloud, and after that things can only improve. Yes I did file a bug report and hope that any spelling errors here will be caught in the Wolfram Community entry panel.
This problem occurs on Windows 7.
I am not noticing either of these issues on OSX. Both seem to work fine: Command-k and Command-;
I don't have an available Windows machine to confirm what you are seeing however.
Bugs happen. Managing the correction of interrelated issues--especially ones that may require non-automated testing--can be complex. (I am not a WRI employee, just making an observation from my own experience, which I know that you also share from your extensive work on user interface development to extend the usability of Mathematica for those who want to use it as their primary means for writing rich documents.)