Thanks all. I wouldn't be surprised if she did have a bit of arthritis. She had in in her hocks about 10 years ago, and had to have surgery to sort it out then. She has been fine since though. I have passed on your comments. Hopefully Mum will get her checked out by the vet.
This sounds just like my daughters old pony, although he was a lot older, mid 30s. Started with a few odd stumbles and then he went right down on his knees a few times when the surface was good and there was no obvious reason. Turned out to be a combination of old age and general weakening, arthritis and back problems (even though we regularly had his back treated), and the vet also found a heart murmur which he thought might be contributing. We retired him as it was too dangerous for him to be ridden, just took him for walks to keep him happy and interested in life, and he lived out his days as number 1 pampered pony! We lost him last year at the grand old age of nearly 40 when he started struggling to get up and looked to be starting to be in discomfort despite medication, and we made the decision to have him PTS before he suffered. Still miss him!
I am sorry to say that it is often a sign of problems but don't stress until you have the vet out and he can give you a full picture which may include treatment. It can be something as easy to fix as a change in how she is shod/trimmed, or a bit of arthritis or something equally simple to deal with. Meanwhile try not to ride her because if she does fall on her knees your mum could have a really nasty accident.
My Mum's pony is a 21 year old welsh Cob. She is perfectly well in all ways, as far as we can tell, but over the last few months she seems to be tripping a lot more than usual. Thankfully so far it has only happened in walk, but she almost had Mum off yesterday when she tripped and almost went down on her knees.
Has anyone else noticed this happening in an older horse? I am wondering if it is a normal part of ageing, or if it could be a sign of some sort of problem?