According to Leo's portrait on the Wall of Honor, he died on November 24, 1942. It's mentioned that Rick and Nathan died on the same day as Leo but when they're looking for where they were buried, Paige announces that they were "buried November 22, 1942, at the local VA." This would mean they were buried two days before they died.
(at around 1 min) When there is a power failure in the house of the Charmed Ones, everything shuts off, even Phoebe's PowerBook. This could only be possible if she had removed all batteries and worked on direct power only.
According to Leo's portrait on the Wall of Honor, he was 18 when he died, however he is visibly older than that and whitelighters don't age, therefore he should still look 18.
At 15.06 when Leo pulls the knife out you can see the attachment that held the blade through the hole in the shirt.
Phoebe goes to the apartment to work on her column in peace and realizes she didn't bring anything to wear for the party that night. This implies she had clothes to choose from to wear to the party. Cole says he'll take care of it and gets her a dress. Later, she is at the manor in the dress Cole got for her after having gotten ready for the party. She didn't need a new dress if she was going to the manor to get ready for the party anyway.
Rick and Nathan have waited 60 years for a descendant of Leo's to go to the reunion. When they see that Leo's "grandson" is coming to the reunion, they are happy to finally get their revenge. In order for this to happen, Leo's first wife would have had to be pregnant when Leo died, and Rick and Nathan would have to have known about it before they died. There has been no mention of a previous child of Leo's, and none of the Charmed ones wonders why they waited so long if they were unaware Leo was a whitelighter until he recognized them.
The charge that Leo is watching over while pretending to be a plumber says that her student couldn't believe that she suspended him. Teachers don't suspend students, principals and other administrators do. She may be the person who went to the principal and got him suspended, but she wouldn't have been the person to actually do it.