I work on a team that supports some equipment related to airplanes. An acronym for one piece of equipment that is decades old is "RCSU". When I got a support call talking about "RSCU", I assumed the person meant "RCSU".

Nope. It turns out, when they made their next-generation piece of equipment, the vendor differentiated it by swapping the inner two letters in an already easy-to-say-wrong acronym.

My reaction was, "WTF didn't they just call it the RCSU2?!"