Blocking meal-time words as negatives. Dinner and lunch look like general dining terms worth excluding, so they get added as broad negative keywords. Planners then search corporate dinner venue, holiday lunch, and private dinner party, and the campaign stops reaching the exact searches it exists for. On event campaigns these words carry two meanings and have to be handled at the phrase level, never blocked outright.
Sending event traffic to the reservations page. A planner needing a room for 40 is not trying to book a table for two. Pointing that traffic at a booking widget converts a qualified inquiry into a bounce.
Judging the channel on cost per click. Event keywords cost more than general dining keywords because the intent is worth more. A higher cost per click on a term that produces inquiries is not a problem to be solved.
Counting form views instead of submissions. A widget that loads is not a lead. If the tracked event fires when the form appears rather than when it is sent, the report will show inquiries that never existed.