At the bowling alley, Grace states, "I suck, but I'm fine with it. This must be what it feels like to be my middle sister." Will even continues this saying "You're actually the baby daughter..." Throughout the rest of the series, though, Grace is actually the middle daughter in her family.
When Rob and Ellen are confronting Grace about how competitive she is, there are several shots of them from the same angle. In some shots there is a red bag visible in the bottom left corner, in others it disappears.
Grace unpacks her Scrabble board game from her bag, and puts it on the table, and puts the bag on the floor. Then her friends tell her she is too competitive, so she goes to her bedroom. As she goes, you can see that the board game on the table has now been replaced by the big red bag.
On the CPU Dummy, the shirt keeps opening and closing.
Before Grace drops the bowling ball down the lane, Ellen said that this was her chance to tie with Ellen, if she knocks down 27 pins... But after Will and Grace go talk about how Will was competitive, Grace scores a spare (which is just 10 pins down) and Rob and Ellen lost. This is not possible.
When Will goes to pour himself a glass of whiskey, the brown liquid that is chosen to represent the liquor has foam bubbles on the top of it that resemble the ones that soda has.