I have heard that some people have a minhag to prohibit boiling just one egg in a pot. What is the source or reason for this? Is this a kashrut concern, or something specific about eggs?
This dinonline answer explains the thinking ... (who knew?)
Some have a custom to boil at least three together.
(The idea behind this is that in case one of the eggs has a drop of blood in it, which cannot be discovered seeing as the egg will be eaten without inspecting it inside, it would be Battel in the majority of two to one. This helps only for kosher eggs which are assumed to have no blood spots. This is not permitted if it is known for certain that one or more of the eggs is not kosher.)