Ctrl+Tab and Ctrl+Shift+Tab can be used to move from a window to the next/previous window. If you click a window's tab button while viewing that window then it will hide itself until you click the button again. Tabbing skips over hidden windows. In 7.42 if you click the window to open it again and tab away from it, you can't tab back to it, as it's hidden again. Infact, if you hide all windows and then unhide all windows (one at a time), and then hide the last one you unhid again, you'll find that all your windows are hidden. The only way to correct the tab order is to close all affected windows and reopen them, and since reordering isn't intuitive you'll probably end up simply restarting mirc.
Query windows opened by receiving a PM can't be tabbed to unless you click to view it once first. In 7.42, it doesn't stay 'open' and you can't tab to it, even after viewing it once by clicking the window's tab button. You have to open the query window yourself.
Last version didn't have this issue.