When Riker tells Worf that he hates surprise parties, and then a moment later comes in and tells him that he loves surprise parties. Was Riker just kidding before when he said he hated them, or did Worf experience his first temporal shift at this point?
It was almost certainly a joke, and not a temporal shift:
From the script:
WORF: Because it is my birthday... I assumed that you or one of the others would try to mount an unexpected... social gathering.
RIKER: You mean a surprise party? Worf, I hate surprise parties. I would never do that to you.
WORF: I am... sorry, Commander.
RIKER: Why don't you settle in and I'll meet you on the Bridge.
WORF: Aye sir.
Of course, then it turns out it was a surprise party:
TROI, BEVERLY, GEORDI, DATA and SEVERAL N.D.S as they rush out of the bedroom. Many hold birthday GIFTS -- others have party favors, noise-makers. It's the most human and obnoxious party you can imagine. They come up shaking his hand, offering birthday wishes, patting him on the back. Troi and Beverly kiss him on the cheek. Riker ENTERS during the middle of all this, comes up behind Worf with huge smile on his face.
RIKER: Actually, Worf, I love surprise parties.
Riker then puts a big, ridiculous PARTY HAT on Worf's head.
Emphasis mine on the word Actually. If it was a temporal shift into an alternate dimension, Riker wouldn't have said "Actually" which emphasizes that what he previously said wasn't really the case.
No, both the script notes (and the episode itself) indicate that changes only occur when Worf is close to Geordi's VISOR.
The continuity "errors" are specifically called out in the script.
GEORDI: (re: painting) Battle of HarOS, right?
Worf looks at him in amazement. Geordi hands a piece of cake to Worf. The interior of the cake is now YELLOW, not chocolate as seen before. (NOTE: Minor continuity changes of this sort will be indicated throughout the script.)
As Worf takes the cake, he reacts to a mild wave of dizziness.
DATA: (continuing) He immediately began to shift to other realities.
BEVERLY (onto idea) And Geordi's VISOR somehow triggered that...
DATA: Exactly. The VISOR uses a subspace field pulse. I think that whenever Geordi came near Worf, the field pulse intensified the quantum flux and pushed Worf into another reality.
Since Geordi hadn't come near Worf before the incident at his birthday party, we can be reasonably sure that Worf was still in the first quantum reality (i.e. the one he shifted to when he went through the fissure) at that point. He then doesn't shift again until after Riker has made his comment about liking surprise parties.