What could happen if the iMac adhesive did not seal correctly at the top of the display? I upgraded my iMac's hdd to an ssd, but the adhesive did not reseal the top of the iMac correctly. There is a gap of approximately 1mm between the chassis and the display. (The sides are fine.)
What is the worst thing that could result from this gap? Should I open the iMac back up and repeat the process?
The worst that could happen is the glass will fall out. I’ve personally seen this happen
Should you repeat the process?  In short, yes.  You want to not only be positive that you’ve got good adhesion around the entire case, but you don’t want to leave gaps where debris, or moisture could get in.   Additionally, airflow is dependent on proper seals; an air gap could cause air to “leak” resulting in inefficient air flow and higher operating temperatures.
The new adhesive and your time is cheap compared to the cost of a new LCD panel.