Some years ago I was flying northish in an area (and country) which I had never visited. This was pre-GPS and so I was combining DR with map reading and leaving a trail of Xs and times where I ws pretty certain I had been.
I ducked under some controlled airspace and by the time I was back to the planned height the map did not seem to match up with what I was seeing. Nothing loath, continue for 5 minutes - obviously I will match things up!
5 minutes further on a very steady heading - total mismatch - I'm over a very large lake - where none is shown.
OK, 180 degree turn and the 5 minutes plus the descent and then the climb to get back to where I was pretty positive I knew where I was.
So all goes well and yes after 10 minutes odd I'm heading south-ish and suddenly the map and the view match. Strange!
So another 180 and this time I concentrate on heading, heading, heading and even compensate for speed. End result - same situation map and view will not match.
So confidence partly restored - course and speed and press on. 5 to 10 minutes later the mismatch starts to vanish and suddenly it all works - 15 to 20 miles odd of map inaccuracy.
End result bang on track and off we go again. Turns out a massive flood prevention scheme had been completed and flooded - so the rver I was following-ish turned into a massive lake.
Lesson DR is fine for short distances, if you can concentrate on it, it works. But big distractions lead you into disbelief and that really messes you up. But in areas with little in the way of distinctive features error checking is difficult.
So if you really want to make it work find somewhere like East Anglia and fly from nowhere to nowhere, mke sure there is a pped or height change - you'll need the GPS to verify you got there but it really does help.
There is a reason that Francis Chichester and that ilk were considered surberb navigators - they were very good at what they did (and possibly a little lucky!)