You spend a lot of time practicing, to improve your playing. You repeat the exercises many times and hope to become a better player. Often it can be the instrument that limits your abilities. Unfortunately, you are not aware of it. You keep practicing hoping to become a better player But what you really do is learn to overcome the weaknesses of the instrument. You waist lots of time to overcome issues that could be fixed where they occur, on the instrument, sometimes even in minutes.
Besides wasting your time, you learned - with big efforts - the wrong things. It's like learning a foreign language from a foreigner, who doesn't have a proper pronunciation, doesn't know anything about grammar, or the appropriate use of many words and phrases. You end up speaking it like him and it is almost impossible to learn it correctly afterward.
To avoid this dreadful circle you need to be aware of it and know about the solutions.
Unawareness of the problem
Being unaware is the main problem. You will end up wasting a lot of time and money.
Unawareness of the possibilities
Without knowing what is possible, you don't know what to expect. It's like bicycling on a byke with too little air - it's very hard, you think that you are out of shape or the byke is bad. You come to the wrong conclusions. Unfortunately, the air is invisible and thats why you often cant see the reason. A leaking instrument will not respond appropriately. It will not produce a full sound, causes intonation problems, and more.
You will try to overcome and compensate for problems caused by the instrument. You might also try to solve it with different mouthpieces, ligatures, or reeds or even a more expensive instrument. Instead of solving the problem, you will end up wasting more money.
Whatever you learn on a fault-free instrument, you can transfer to any other instrument and it will work. If you need to learn the awkward trick to do that, they might not work on anything else.