Ship of Theseus
The Ship of Theseus is a development "trap" that often bites new RepRap developers.
Say you want to create a the cool-new post-Mendel RepRap. "I'll do a delta-type reprap controlled by a 32 bit ARM processor with brand new driver software ...". There are two ways to go about this.
Reinvent the RepRap from scratch, developing and fabricating each new subsystem. Then try to get everything to work. Note that you do not actually have a working 3D printer yet to use as a model / and to use for fun.
This way you have a working 3D printer to bootstrap the new and improved 3D printer.
This way is much more fun.
The Ship of Theseus, also known as Theseus' paradox, or various variants, notably grandfather's axe (US) and Trigger's Broom (UK) is a paradox that raises the question of whether an object which has had all its component parts replaced remains fundamentally the same object.
According to Greek legend as reported by Plutarch,
The ship wherein Theseus and the youth of Athens returned [from Crete] had thirty oars, and was preserved by the Athenians down even to the time of Demetrius Phalereus, for they took away the old planks as they decayed, putting in new and stronger timber in their place, insomuch that this ship became a standing example among the philosophers, for the logical question of things that grow; one side holding that the ship remained the same, and the other contending that it was not the same. —Plutarch, Theseus