List of tricky words to spell according to Oxford Dictionary:
Otorhinolaryngological: relating to the medical practice involving the ear, nose, and throat.
Pseudopseudohypoparathyroidism: Inherited disorder, named for its similarity to pseudohypoparathyroidism in presentation.
Floccinaucinihilipilification: the estimation of something as worthless.
Pneumonoultramicroscopicsilicovolcanoconiosisan: invented term said to mean a “lung disease caused by inhaling very fine ash and sand dust”, but rarely used except for its curiosity value