failing to shut off the plasma cutter has very little effect (other than possibly a small amount of damage to the piece being cut), if you go off the edge, the plasma isn't going to be sustained, if you leave the cutter in place, the plasma will eat a small amount around the location until the circuit can't be sustained any longer and the plasma will cut off.
These things rely far less on precise timing than you think.
Just about all of the DIY devices rely on stepper motors, which move a specific distance when pulsed, not normal motors run for a specific amount of time.
If the pulses are late, things move a little slower. If there is enough momentum in the system, it's possible for that momentum to cause the equivalent of 'jumping a tooth on a gear' and being slightly out of position, the solution to this problem is to slow the machine down a bit.
synchronization between different pieces is a matter of either moving one motor, then a different motor, then the first one again, or in setting up the movement for both motors and sending a 'move now' pulse. In either case, slight delays don't break anything.