What did Albert Einstein invent by accident?

Answer 1

Nothing -- he was not an inventor, but a theoretical physicist.

None of his theories came from accidents. He would see a common phenomena (a clock advancing in time as he rode a trolley, or a person working on scaffolding), think about the physical significance of it (What would I see if my trolley was moving away from the clock, faster than the speed of light? How would a falling person experience the laws of our Universe?), and then spend YEARS working out the mathematics of these thoughts. For example, the start of general relativity was during his "happiest thought" in 1907; his final paper on the subject came out in 1916.

His theoretical work DID allow him and Leo Szilard to develop a plan for a refrigerator that had no moving parts, which they patented in 1930. It is unclear if Einstein did any significant scientific work on this project. The rights to this concept was purchased by a Swedish company, but is has never resulted in any major, practical appliances.