I'm a dark-ages re-enactor in my spare time, and the biggest event of the year is always
the Battle of Hastings. I was in the front line (a poor Saxon conscript) as one of the Norman cavalry charges came our way.
One of the Norman knights lost control of his horse and it started veering into our shieldwall. Everyone was shuffling back and out of the way, but the hersir (someone who relays commands up and down the field) standing in front of our line was looking the other way. The horse collided with him from behind and he went face-down into the mud. Blam. Blink and you'll miss it. I don't think the horse slowed down at all - it just kept going until the knight regained control of it.
We rushed forward to check on the hersir. His spear had snapped in half, his shield was pretty beaten up (and had awkwardly twisted his arm), and he was barely moving - shock I think. He also had a bloody nose where his face had received the good news from the ground - I assume that the nasal guard of his helmet helped with that.
Fortunately, he avoided sustaining any serious injuries. He hardly knew where he was or what was going on while he was moved off the field. I heard he was suffering only bruises and some concussion after the battle - no broken bones. I guess a hauberk isn't really going to do much against blunt trauma like that - I imagine he had circles all over his back where the mail had been pressed into his skin.
I've no doubt that, in later centuries than the one we were re-enacting, brave pikemen would stand their ground and make horsemeat kebabs out of enemy cavalry; but if it was up to me, I'd prefer to jump out of the way and hack at the rider with a longaxe or a big sword.
Of course, the horse isn't the only thing you have to worry about if the rider has a lance or something. Best to gang up on a mounted opponent, and hope that at least one of you has a big shield.