Selling an uncovered call is the one common options position with no upper bound on loss. Stocks can double overnight on a bid; indices cannot, but individual names absolutely can.
Because of that, brokers require top-tier option-approval-level, impose the full naked-option-requirement, and often raise it around earnings. Most traders who want the same exposure use a credit-spread or a covered-call instead.
Example: XYZ at $50, sell the $55 call for $0.80, collecting $80 against roughly $560 of buying-power-reduction. XYZ is acquired at $75. The call is worth $20.00. Loss is $1,920 — more than three times the capital the broker had set aside for it.
Related: naked-put, undefined-risk, naked-option-requirement, covered-call