The purchase order system you need to build should be easy.
All the user has to do is select a supplier from the list of registered suppliers that you deal with, then enter a series of parts, descriptions, and quantities, and you're done. Print it off, mail it, wait for the order to be delivered.
You write the code in a week, and you sit back, waiting for praise and thanks to shower on you for the wonderful job you did.
What showers down on you, instead, is fifty shades of batshit
Users get half-way through entering an order, then notice a mistake on one of the earlier lines. But you gave them no way to go back up the screen so they have to scrap it and start again.
Users start, then realize they need to look up a product code, but they can't leave an order half-finished. They have to scrap it and start again.
It gets worse if you do reach the end of your order, because there's no opportunity to review it before it gets automatically printed off, or electronically faxed if the supplier is set up that way. Because you were being quick and efficient and decided to save them precious time. Orders are flying out the building with mistakes in them, that your company is being invoiced for by your suppliers.
And just because an order was correct at the time doesn't mean it will stay that way. One supplier is out of stock so you need to re-order either an alternate part or from a different supplier. But your system is expecting all goods ordered to be delivered. There's no means to cancel an order once placed.
Then the supplier only delivers five of the eight items ordered. The rest will follow next week, but you assumed it would all be delivered together and didn't think of part shipments.
All this adds up to one simple lesson...
Coding the happy path is typically less than 10% of the work in developing a system. If you only work on the happy path to save time, you'll be in a world of hurt later on.
Dealing with the unhappy paths should be the lion's share of your effort, and doing so gracefully marks the professional from the amateur.