I have a 12 yo Maine Coone type (not sure if he's full MC) boy cat, Ivan. We have had him since he was 2. He's been an indoor kitty all along and has been generally healthy. He used to be overweight, but we have slimmed him down a lot, he's still a big boy at 17lbs. He's generally pretty active and has spiral stairs and cat trees to climb up and down. We do discourage him from big leaps though. He's always had a bit of a funny waddle, being a bit cow-hocked. He has a slight limp from time to time in a front leg. Since he's a big boy, and aging, I expect some big-cat health issues that MC cats are prone to.
This evening he was laying on the kitchen floor while I was cooking. He got up to move and it seemed that his hind end wouldn't work, it seemed to be his right hind leg was extended and wouldn't move. He didn't seem to be in pain, just his leg wouldn't work. He stumbled around for a few moments. I checked him over for a few minutes, and soon he was back to normal.
In a panic at first we tried calling our vet to see if they had an ER, but it is a holiday and no answer (7/4).
In the past hour he's back to walking around as normal, had a good drink and is now up in his favorite window perch.
My bf will call the vet tomorrow to get advice.
Looking online I'm guessing a
luxating patella. I'm familiar with this from when I worked with horses way-back-when. He maybe has some mild hip displaysia issues too, being big and cow-hocked, though he doesn't seem in pain.