... picking up the game right at the point you caused a fall-out, or at least a checkpoint nearby, really would have helped me to overcome the final, somehow aslope section of RDF20 much easier (without gathering so much frustration while having to go through 'the whole narrow nightmare' over and over again...) ...
On second thought, maybe having to clear a part of a level over and over again to practice a hard section was something that the original developer(s) intended? This would mean that a practice mode as I am envisioning would (sort of) break a premise of the game. After all, learning to suppress your frustration in such cases builds patience.
Otherwise, maybe another solution to "frustrating repetition" is in the level design. Example: My level "Bounce Box" is, in my opinion, a tiring pair of elements: first you have to collect quite a few coins to unlock the goal. To do this, you either follow a tedious method, rolling up the same halfpipe several times and carefully falling straight downward to collect coins. Or, you use the launch pads placed in the level to collect some hard coins, which is about Nevermania-difficulty. Next you potentially have to "re-activate" these launch pads, which is also tedious. Then, to reach the goal, you have to scramble to make several difficult jumps using the launch pads (difficulty spike from previous actions !!!). If you fall-out here, you have to do the tedious part all over again.
The line of events of RDF 20 is also linear, with easy moves followed by one or more hard ones. This seems to be a design trend in many levels (such as in TDF and RDF, Nevermania, Ctoan's levels, etc.). Basically what I meant in the example using my level is that levels with forking/forking-and-merging/branching/etc. lines of possible actions is a way to avoid "frustrating repetition." (Some levels like this: Medium 9, Medium III, TDF II)
Now I feel ambivalent.
--It the it
P.S. To fix the problem in Bounce Box, I plan to separate the level into 2 non-bonus levels: one with the coin-collecting part and the other with the scrambling-and-jumping part.
It the it's level set: http://neverforum.com/fmpbo/viewtopic.php?id=2922
6/19/17: a bit of help needed here, plz: http://neverforum.com/fmpbo/viewtopic.php?id=2988
I created my avatar with ANVIL-1000MD (c) and scaled it down with GIMP.