The problem with the question about traveling at lightspeed and all of the answers to it so far is that traveling at lightspeed is impossible. The answers so far have all been about what happens when you travel at significant fractions of lightspeed, but that's not the same thing as traveling at lightspeed itself, so it's not what the question was about. For that matter, the effects they're talking about for an object moving at a significant fraction of lightspeed are the reasons why you can't get to lightspeed; you can keep speeding up forever and still not reach it because space and time stretch and squeeze in a way that conspires against you; you can always decrease the difference between your speed and light's to a fraction of what it was now, but never down to zero.

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A solar sail can easily be on a collision course with the sun, if it's coming in from outside the solar system at a high speed, trying to use the sun as its brakes, and for some reason not getting enough braking power from it. But then, they should be able to veer enough to one side by tacking...

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The worst astronomy I've seen myself was in a movie from the 1950s or 1960s (as most of the worst science fiction tends to be), but I don't remember the title. It featured another star and its planets suddenly, surprisingly crashing through our solar system, with various planets getting either destroyed/marred or relocated. Humans survive by hopping from Earth to a planet that came with the other star, which ends up orbiting the sun.