As Doc Savage writes, it's likely to be human error.

The biggest culprit in the hotels in which I worked were batch check-outs. Since lots of guests don't formally check out, it's up to Housekeeping to inform the Front Desk when they find vacated rooms. Unless there are tons of early arrivals waiting for rooms that day, Housekeeping typically creates a list that gets delivered to the Front Desk for processing.

Sometimes, Housekeeping will have time to actually clean the room before it gets checked out in the system. In that case, they will submit a separate list of clean, vacated rooms. If something goes wrong there, you're more likely to run into this problem.

Whether done by hand or electronically, that gives you at least two different people who can make simple entry errors when recording room numbers. The computer will warm the Front Desk if the departure day is wrong. But, if the correct room and the wrongly-entered room are both due to depart, it's possible the error will not get caught.