This is the hardest to cope with, because, if it hits, there is absolutely nothing you can do to get your macro going again from the point at which it stopped, you will have to start it again. You can't trap the error with the On Error command. The only thing you can do is to take avoiding action. If you are running Word 6 rather than Word 7, and you have a subroutine which is called very frequently, convert it to a function. You can throw away the return result.
Word has insufficient memory.
You will not be able to undo this action once it is completed. This is a bit easier to cope with, even though it happens more frequently. It is caused by the Undo buffer filling up. There are very few single actions which you can take which will trigger this. Word can undo the last 100 actions, but it needs to store somewhere what those actions were.