^Not unrealistic, just (very very very VERY) statistically improbable...but still possible.
Just to make it even more unlikely though, even *if* a probe found it's course altered by a (relatively) near encounter with a gravity well (like an asteroid for example) and found itself heading for a planet, the odds of it not getting pulled in and consumed by either a Jupiter-like comet sweeper or the star itself are even narrower. On top of that and assuming it isn't just catapulted back out into interstellar space or obliterated by a dense matter cloud, planetary ring of solar eruption and *if* it does make re-entry on a rocky planet and *if* it doesn't burn up on the way in, chances are it just landed on a dead rock that never had a chance to bare life.
In shows like Star Trek the usual conceit is that intelligent life everywhere and there's been countless interstellar civilizations knocking around for billions of years at one time or another so the Enterprise is always tripping over their old junk. In reality, it's far more likely that any civilisation that encounters an alien probe does so deliberately. Either sent after they were remotely detected or a TMA-1 type devise that is actively on the look out for life forms and has billions of years to burn looking for them. In either case unless the other race is malicious, it's almost a certainty that they'd have taken enormous precautions against contamination.