However, if you want an example of a time when someone went looking for one thing and found something entirely different, you might look up the history of the Post-It Note.

From wiki:

“Saccharin was the first artificial sweetener and was originally synthesized in 1879 by Remsen and Fahlberg and its sweet taste was discovered by accident. It had been created in an experiment with toluene derivatives.”

From Elmhurst.edu

“Saccharin was discovered in 1879 by Constantine Fahlberg, while working in the laboratory of Ira Remsen, quite by accident as were most other sweeteners.

While working in the lab, he spilled a chemical on his hand.

Later while eating dinner, Fahlberg noticed a more sweetness in the bread he was eating. He traced the sweetness back to the chemical, later named saccharin, by tasting various residues on his hands and clothes (unsanitary conditions) and finally chemicals in the lab (not a safe lab practice).”

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I think by spilling the chemical on his hand it counts as a failed regular experiment.