If you took one bucket of water from a small pond, it really wouldn’t make that much of a difference.
But hundreds or even thousands of buckets?
Now you’ll be making a difference.
Likewise:
One bad performance doesn’t erase many good performances.
Missing something once doesn’t erase being there every other time.
Being undisciplined once doesn’t erase being disciplined every other time.
If you’re consistent most of the time, these blips just won’t matter. They won’t happen often enough to even matter.
The key is that they’re infrequent, though.
If these little blips become regular, well then now it’s going to start to matter.
Keep them infrequent and you’re fine when they do happen.