Ruby's begin-end block control structure is often overlooked but can have unexpected effects when combined with conditionals like `if`, `while`, and `until`. When used alone, a begin-end block doesn't change the behavior of the code. However, when paired with these conditionals, it can alter the execution path of the program. In one example, using a while loop with a false condition resulted in the code within the begin-end block being executed, despite the conditional checking for falseness. This demonstrates Ruby's do-while construct, which checks the post-condition after processing the body of the loop. The behavior can be surprising and may lead to confusion among developers unfamiliar with this syntax.