My steering wheel suddenly moved left in my hands - but the car kept
going where  I'd been pointing. I was driving my 96 Mazda 626 on the
interstate, at about 70mph, on a gradual curve. I think I hit a small
bump, maybe an expansion joint,  when suddenly I heard a small soft
thunk and the wheel moved. Alarmed, I pulled over to the  breakdown
lane, slowed down to about 30, put on my flashers. But the car seemed
to steer just as well as before, only now the wheel was cocked 45
degrees left.  I drove home about 10 miles on back streets going slow
with flashers on, but  there were no more events, steering was fine.
This happened on Saturday afternoon, and I haven't been able to get a
mechanic to look at it yet.

There's a little steering gear context which may or may not be directly
related. A month ago (only about 150 miles) it needed a new lower right
ball joint to pass inspection. As soon as that was done I noticed soft
thunking/clicking from the gear when turning corners. The mechanic took
a close look and said it was my upper strut mounts and bearings. The
car has 159k miles. I got a little demo as he had me feel the bearings
as he turned the wheel. He said a $300 job. I asked if it was the sort
of thing that could result in
a sudden loss of steering or a real safety concern. He said it was more
like the gradual wear of brakes or a clutch, that I'd be able to sense
when it was getting bad, and meanwhile I could drive if I just put up
with the creaking on corners. That was only about 75 miles ago.

Now I'm curious to get an idea about some things.

What kind of failure could cause this and still let the car drive fine,
at least for 10 miles at 30mph?

If you have an idea of that, maybe you have a ballpark idea of labor
hours?

Any other comments or info are welcome.