I was helping a member on here change pads...And while he and I were removing the pads...I noticed that on his 07 GS350 his caliper was seized on one side.. (while one axle had newish pads) the other side was done for...Any possible culprits? the car has 65k miles I believe.

 

It can happen but isn't hard to rebuild. Could be due to the fluid never being flushed or too much moisture in the fluid itself causing rusting leading to seizing.

 

I happened on my matrix. Look at the sliding pin, it's a cheap part and may even just need to get banged loose and greased up.

 

There are no pins on the GS350. They are fixed calipers, not floating like GS300's. But you're right, seized pins can cause it too on floating calipers.