This sign is funny in that it states the obvious problems associated with a sign advertising a temporary issue. JOKE EXPLAINED.
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 because a warning of this nature is largely superfluous. JOKE EXPLAINED.
This picture is funny because the person who made the sign was clearly frustrated enough about this problem to make a sign about it. JOKE EXPLAINED.
The original sign creator being 'shut down' by the second sign creator in such a forceful manner is what causes this exchange to be funny. JOKE EXPLAINED.