An example from chemistry: get a mixture of different chemicals in a lab class that you are tasked with separating and identifying. Screw up one separation step

An example from chemistry: get a mixture of different chemicals in a lab class that you are tasked with separating and identifying. Screw up one separation step and lose one or more of the chemicals, you aren't getting it back. If you're lucky maybe you can take the NMR spectrum of the whole sample, isolate out the contributions from the other chemicals you were able to separate and then possibly be able to identify the missing one, but you're not going to be able to turn in an isolated sample of it so that'll at best cost you a bit. An example from math: you make one tiny logic error while composing a proof and miss it until you start rereading, then you might have to start all over again from scratch.