You know when you're in for a strange game when...

read the title post your answer.

A cat deep strikes your army.

... The chaos player sacrifices one of his minis before the game to worship Khorne...

you field an all infantry army against empire and they only field infantry as well

true story!

When your Skaven opponent begins chittering at a nearby wall for advice and gives his army a speech before the game begins.

This!

You play outside in the garden under a tree and the wind sets loose an orbital bombardement of leaves and fruits on your battlefield.

Greetings
Eta

When two turns in a row you miscast and roll double ones on the table.

Your TK opponent starts deployment with 40 archers in a single rank across the front of his deployment zone.

The opponent's gunline shoots down two Stegadons before they can make impact but is then tied up by the pair of surviving skink chiefs until you can turn his flank.

...things seem to be going my way.

Ha! Unlikely.

Your dwarf opponent doesn't camp with his warmachines on the hill and his 6 unmovable blocks around it. The bloke marched across the table and met my druchii army in the middle (well, I was a bit faster ). He had a single warmachine and only one thunderer unit, the rest all CC. One of a kind I tell ya!

When the DoCplayers GuO with Trappings dies from shooting and magic before it gets to move.

(Happened in a DoC vs HE game, the GuO got killed by 2 RBT's, 20 Archers, 1 Fireball and 1 Fireblast in the HE players first turn. Yeah, it was some wicked rolling from the HEplayers part and some really bad rolling from the DoCplayer.)

Your Tzeentch exalted bsb on disc with flail fails routing a 10 marauder on foot unit, despite 4 S7 attacks and flank charge, and gets stuck for 4 turns. Then you've to charge 5 crappy horsemen on the rear of the same unit to unblock the situation.