I must be going blind or I've been staring at too many special rules lately. Where does it say what happens if you fail an acrobatics check to move across a narrow surface? Acrobatics talks about failure to move through an opponents square and failure to jump, but does it actually say anything about failing the check to move across a narrow surface?
I was expecting to find something like: Fail by 4 or less, stop moving, fail by 5 or more fall.
It's more implied than anything, except if you're making the check from taking damage...there it actually says you fall on a failed checked.
Check: You can use Acrobatics to move on narrow surfaces and uneven ground without falling. A successful check allows you to move at half speed across such surfaces—only one check is needed per round. Use the following table to determine the base DC, which is then modified by the Acrobatics skill modifiers noted below. While you are using Acrobatics in this way, you are considered flat-footed and lose your Dexterity bonus to your AC (if any). If you take damage while using Acrobatics, you must immediately make another Acrobatics check at the same DC to avoid falling or being knocked prone.
So, if you fail the DC on uneven ground you go prone, but if you fail on narrow surfaces you fall off is my reading.
I was wondering about crossing narrow spaces too. So there are no checks to see if you catch the ledge?
I've encountered this situation in a number of games and I've yet to play with a GM that didn't allow a Reflex save to catch the ledge.
Also, can you crawl across it or take full-round, five foot moves for a bonus? The rules seem a bit limited.
Yes you could crawl just as you can when you're prone, but movement is limited to 5'/round. You can't crawl and take an additional 5 ft step as you're never allowed to take a 5 ft step if your PC has used a move action to actually move - crawling is a form of movement.
This sort of thing just requires common sense, which is why it's not explicitly stated in the rules. If there was a rule for every situation like this the CRB would be a 1000+ pages instead of the 575 that it is.