Its possible that it could be something else. Since you've only experienced it once so far I wouldn't worry about it. I was just giving you some things to be aware of to aid you in determining exactly what the problem actually is.
25k miles is not a lot... clutches and bearings should easily last 5 times that long... However, one thing that does cause throwout bearings to fail prematurely is parking at a stoplight with your foot on the clutch and your transmission in gear. Its much better on the throwout bearing if you place the car in neutral and remove your foot from the clutch while the car is not moving.
Like I said, just drive normally and dont worry about it until it starts happening more often. Probably too hard to make an accurate diagnosis until the problem starts to present itself more often.
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1985 300ZX n/a
See my Z project