Garmin 705
When you set off press the start button and you get the message "timer started" then at the end press stop button and you get "timer stopped".
if you create the laps in wrong places , i.e. you press the button more than once don't worry you can join and cut the tracks afterwards.
all the detailed info is retained on speed etc.
I think it all stays as one file until you either reset the 705 by pressing and holding the 'Lap' button for more than 3 seconds, or by plugging it into a PC, at which point it saves the history automatically. (There are threads about getting this occur with some chargers.) If you have not done either, then you still have lap information available on the .tcx or .gpx file, but everything is still in the one file, and you should not need to join tracks afterwards. This is also the case with Garmin Connect.
Being a bit daft, sometimes I am riding along and have so many items showing on each screen that the timer only shows hours and minutes, and you can convince yourself that maybe you didn't actually start the timer, after the last stop, and of course you can't possibly wait up to a full minute to see if the minute number changes. A quicker way to check is to spress the start stop button, and if it says timer started, then say "Bugger" very loudly, and hope that you haven't missed too much. However, if it says timer stopped, then you press it again immediately, and all is well.
Another way you can be sure it is actually timing is to look at the 'grade' window, (if you have one) - it only shows a number here, if it is timing.