In one particular episode, one of the actors accidentally refers to Doug Heffernan (of King of Queens, as played by Kevin James) as "Kevin" - his real life name. Kevin James appeared 9 times in "Everybody Loves Raymond." In the first six appearances, during seasons one and two, his character was simply referred to as "Kevin." In the final three appearances, during season three, he portrayed his "Doug Heffernan" character from "The King of Queens." This is probably because the final appearance as "Kevin" took place in May of 1998, while "The King of Queens" did not debut until September of 1998.
In Season 2, Episode 25, Raymond meets Debra's priest Father Hubley, in a flashback episode featuring Ray and Debra's wedding. However, throughout the rest of the series, Father Hubley is portrayed as the Barone family's priest. This portrayal includes the Season 6, Episode 24 flashback episode during which Marie brings Father Hubley to Debra's house long before the wedding.
In the episode with Raymond's diary, Marie makes like she is reading from Raymond's diary, however from the angle that she's holding it, it's obvious that there's no writing on the page except for a standard header on every page. Raymond also doesn't write anything when she asks him to amend the entry.