personnally, I've have 2 broken mounts on my 93 SSE and one on my 93 SSEi. The SSE was the rear tranny mount. The SSEi was the front lower mount.
I am not sure why mine happened. could be my driving style (was 16-19 when I had the 93 SSE). I can't say they are really common for breaking, but it does happen. You'll usually feel excessive movement of the motor when they start to go. If you have the hood up, and the car on, you can press the throttle and feel/see the motor move. that'* how I figured mine out.
the one on my 93 SSEi got bad cause I let it go for over a year and it started causing shifting and idle issues, but when it was replaced, it fixed it.
2007 Saturn Aura XR | 1970 Chevy Blazer 4x4 | 1970 GMC 3/4 Suburban 4x4