Is it subtile or subtle humor?
Not to risk derailing this thread (again) but apparently the retired US Navy Petty Officer is correct in his usage (if a bit dated):
subtile
Main Entry: sub·tile
Function: adjective
Inflected Form(s): sub·til·er
Etymology: Middle English subtile, sotil subtle
Date: 14th century
1: subtle elusive <a subtile aroma>
2 a: cunning crafty b: sagacious discerning
Archaic spelling of subtle
subtle
Main Entry: sub·tle
Function: adjective
Inflected Form(s): sub·tler; sub·tlest
Etymology: Middle English sotil, subtile, from Anglo-French, from Latin subtilis, literally, finely textured, from sub- + tela cloth on a loom; akin to Latin texere to weave — more at technical
Date: 14th century
1 a: delicate elusive <a subtle fragrance> b: difficult to understand or perceive : obscure <subtle differences in sound>
2 a: perceptive refined <a writer's sharp and subtle moral sense> b: having or marked by keen insight and ability to penetrate deeply and thoroughly <a subtle scholar>
3 a: highly skillful : expert <a subtle craftsman> b: cunningly made or contrived : ingenious
4: artful crafty <a subtle rogue>
5: operating insidiously <subtle poisons>