When flyspell-mode is enabled and I try to write in Greek, I get an error most of the times I try to type an accent above some character:
<268632064> is undefined
The normal sequence to type a character with an accent is to first type ', followed by the appropriate vowel. Instead, whenever flyspell-mode is on, ' and the previously typed character are interpreted as a call to <268632064>, whatever that may be, which triggers an error, and, thus, the following character appears as is without an accent.
So,
when flyspell-mode is off, λ ' ε -> λέ
when flyspell-mode in on, λ ΄ ε -> λε and an <268632064> is undefined error
Note: I'm using GNU Emacs 25.3.1 (x86_64-apple-darwin16.7.0, NS appkit-1504.83 Version 10.12.6 (Build 16G29)) on a mid-2015 macbook pro.