Continuity mistake: After Jack and Karen ask Grace if she knows any good designers, they run over to the table and grab a pencil each. After they grab their pencils, the position of the pencils in the pots changes. (01:06:35)
Continuity mistake: When Grace is looking through the sheets pulled from the magazines which Jack gives her, the paper at the back of the pile she is looking at changes between shots. You can tell because the layout is different. (01:09:30)
Continuity mistake: When Grace is in the batting cages, she gives Will some money to go buy her a corn dog and some nachos. The door to the cage is wide open, but in the very next shot of Will, the door is locked shut. (01:17:50)
Answer: The director, James Burrows.