do..end vs {}: Not just a matter of style!

“Use curly braces for single-line blocks and do..end for multi-lines blocks.” Having seen various permutations of that statement dozens of times, I came to believe that the difference between the two was purely stylistic and that there was no practical reason to opt for one vs. the other. Most of the time, that is, indeed, true. Until that one-time-in-a-thousand when it’s not.