What do you call the behaviour where someone is a bit too hasty and takes an action without any preparation
For a very specific example, say you have a software developer who makes some changes to a product, then does no testing of the changes and then releases the product only to find that the customer finds the product no longer works. The person is too hasty and not diligent enough to follow a careful process.
Solution 1:
Three words come to mind. All have connotations of haste and unfortunate consequences.
Precipitate = If an action is precipitate, it is done sooner or faster than expected and without enough thought or preparation “Don't be precipitate - think it through before you make a decision.”
Cambridge dictionary
Impetuous = likely to do something suddenly, without considering the results of your actions
Cambridge dictionary
Rash = careless or unwise, without thought for what might happen or result
Cambridge dictionary
You may also like to research foolhardy and (for a bit of pretension) temerarious, although I think them slightly less suitable for your specification.
Solution 2:
The person's work is slipshod. Lexico has
slipshod
ADJECTIVECharacterized by a lack of care, thought, or organization.
Having witnessed the slipshod management here, it is not surprising the programmes they run result in little success.
This covers the OP's specific case where the person isn't only hasty:
The person is too hasty and not diligent enough to follow a careful process.