Final answer:
Observations of Supernova 1987A were validated by multiple telescopes, while lack of reproducibility hinders acceptance of the fifth force experiments. Forefront experiments are more susceptible to observational problems compared to those involving established phenomena.
Step-by-step explanation:
Observations of Supernova 1987A were valid because they were confirmed by multiple telescopes, providing consistent data. Although the event itself is not reproducible, the observations made during the event were reliable and meaningful.
The lack of reproducibility for the fifth force experiments is due to the nature of the phenomenon and not necessarily poor-quality experiments. Forefront experiments are more subject to observational problems compared to those involving established phenomena because they often deal with complex and unpredictable events that are harder to control and reproduce.