Nerd mode engaged.
It was a tape demarkation on the floor, over five meters long, not perceptibly diverging from a line between floor tiles it was taped over, a line which I have perceived as straight whenever I perceived myself to be sober. (And no, I did not put the tape there myself, nor was, to the best of my knowledge, the person putting it there, inebriated or of malicious intent of deceiving me or other patrons of the celebration.)
I trust the tape and the tiles used to conform to the applicable DIN norms or similar, and furthermore maintain that a line which has an imperceptible divergence from straightness is, in the context used, as good as straight.
:p