Knee/patella has a catch?

Something's goofy in the past 24 hours. I sometimes can't put my left leg straight out in front when walking; my knee catches.

Right now it's fine, but if I cross my legs or get up after sitting then try to go up/down stairs or just walk, it's as if the tendons or ligaments on the front of the knee aren't working, or are hanging up on something: a mechanical blockage just inside the knee below the patella (superfical anterior).

It's like a 'crazy bone' when I move it forward (no weight), or just as I start to put weight on it during a step, and even at that not all the time. Never had this symptom before. I'm 50, 5'9" and 160 lb.

Possible cause of injury is yoga--downward dog, or sitting against a wall (to strengthen quads for skiing). I did both yesterday, and I got a cramp in my knee at yoga when we were doing dolphin pose. Now I have this walking issue. Nothing is swollen.

Your Patella might be misaligned, probably beacuse of uneven muscle pull due to uneven muscle stregthening. The inner (medial) part of your quadriceps muscle is probably relatively weaker than other parts of your quadriceps muscle (which has 4 parts). I'm a Physical Therapist and most of my patients have the same symptoms as you. You probably have a patellopafemoral pain syndrome
Because bending the knee increases the pressure between the patella and its various points of contact with the femur, patellofemoral pain syndrome is often classified as an overuse injury.4-8 However, a more appropriate term may be "overload," because the syndrome can also affect inactive patients. Repeated weight-bearing impact may be a contributing factor, particularly in runners.3 Steps, hills and uneven surfaces tend to exacerbate patellofemoral pain. Once the syndrome has developed, even prolonged sitting can be painful ("movie-goer's sign") because of the extra pressure between the patella and the femur during knee flexion

take an aspirin and call your doctor in the morning

Could also be the meniscus cartilage behind the knee that is weak and or torn too. Any Pain or weakness when climbing the stairs?

I would not ignore knee problems. They have a way of getting worse as one ages and before you know, walking can be a problem. I would go to my physician and and have it x-rayed - just to find out what is happening with it. A doctor is better able to diagnose it after and the doctor will provide the solution. If you know the condition you can also research it on your own to find out what others have done. First step is to find out what is happeing with the knee and only a physician can help with that.

Locking is a serious symptom in knees and does point to some mechanical blockage within the joint.

My thought, and likely the worse case scenario, given that it happens after crossing your legs, is a meniscal tear. This usually manifests as locking of the joint and pain along the actual joint line (usually to the inside).

However, there is also a very common condition, one I've had myself, which comes to mind when you mention pain and problems going up and down stairs, or after rising from a seated position. Chondromalacia patella (a.k.a. patellofemoral syndrome) is incredible common in women, and is caused by bad "tracking" of the kneecap in the groove formed by the femoral epicondyles. With time, you can actually form degenerative arthritis of the underside of the kneecap.

Go see your doctor. Most should be able to easily diagnose patellofemoral syndrome with aa good exam and possibly some x-rays. If they aren't, then ask for an MRI of the joint to rule out the meniscal tear.