If you tell someone that you’re not able to commit to something, that’s a good thing.
If you tell someone that you’re able to commit to something, then later discover that you can’t, that’s not good, but you can still update them with your current status, and at least they know.
But what’s even worse is not updating your changed status as soon as possible. Because in that case, the other person is still relying on you – and you’ve essentially gone silent.
That last one burns bridges.