I mean there is always two sides to every story. Obviously you will tell your side in a way that people will agree and side with you. You must have made some pretty bad errors for your PI AND Grad student to point them out. I personally think it's his lab and he can hire or fire whoever he wants. If the PI feels like you are making mistakes (which you were, more than one mistake) that are holding his lab back, he really has no reason or incentive to keep you there. You have to keep in mind that the mistakes you make in lab COST MONEY. The reagents you use to carry out the experiment as well as the equipment is definitely not cheap. And labs are run by grants the PI worked hard to get. So your mistakes as small as they may seem to you, cost the PI grant money. In the end you're just an undergrad, not a grad student. So he's really not missing out on anything by firing you. It might be even better for him and his lab to fire undergrads that make mistakes. You're being extremely entitled and arrogant by expecting him to keep you.