Probably the most common place where to encounter it: do we agree A major is a fairly common key in pop? Isn't F# minor its relative minor? Well, the latter's dominant chord is C#7, whose 3rd is E#, no two ways about it.The 7 in the key of F# major or the 3 in the key of C# major is an E#
See above. I concede common R'n'R rehearsal room parlance would probably spell that note F, but it would be incorrect.It sounds like the guitarist messed-up. True that E# exists, though you have to go a long long way to find the word used, particularly in a standard band rock band.
Of course, having context would decide if note names as given to the OP are/were appropriate.

