In some Ashkenazic dialects, lots of words in which the first syllable is accented and the vowel is segol are pronounced as though they had a tzeirei. Examples include "meylech," "tzeydek," and "peyrek."
But even those who pronounce words of this class with a segol (like me) still say "peysach." As far as I know, this is the only one where this is done. Why is this?