Trust me on this, I have personal experience. Just because you have a plan and you checked it twice, something can always happen.
I have a car that of course comes with monthly payments. I plan ahead every month for it so my credit isn't affected (we will learn about that later). As I was making the payment the app glitched so I had to repeat the whole process over. When it came out of my account a couple of days later, it came out of my account twice. TWICE! So I just paid two times for my god damn car. This car better last awhile. I mean it's still paid....right?
The moral of the story, at least this part, is that just plan for the worst. As negative as that sounds, it works. If I hadn't had extra money in my savings account....my checking would be in the negative.