The Grace and Karen getting stuck in a shower is a nod to The Lucy Show episode "Lucy and Viv Put in a Shower."
Since this episode revolves so heavily around the concept of age and aging - Eric McCormack (Will) is actually seven years older than Sean Hayes (Jack). At this point in time, Will is well over 50 years old.
Mary Pat Gleason also appeared on Acting Out (2000) and Sweet (and Sour) Charity (2000). However, she played a different character.
Will uses a white monogrammed coffee mug bearing the letter W in a black font. The same, or similar, style of mugs can also be seen being used by the main characters of Kevin Can Wait (2016) and Fuller House (2016).