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 less funny and more a statement about how much the sign owner values sarcasm. 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 funny contextually because usually when someone is memorialized it's because they love that place or thing. Not so with this plaque. JOKE EXPLAINED.
This sign is funny because a warning of this nature is largely superfluous. JOKE EXPLAINED.