The 'sorry' addition is what is funny about this sign. Signs are designed to state information clearly and professionally and the 'sorry' gives the sign a human element that is uncharacteristic. JOKE EXPLAINED.
This sign is funny in that it states the obvious problems associated with a sign advertising a temporary issue. JOKE EXPLAINED.
This sign is less funny and more a statement about how much the sign owner values sarcasm. JOKE EXPLAINED?
This sign is funny because a warning of this nature is largely superfluous. JOKE EXPLAINED.