*I've seen that happen a number of times. Sometimes very old service
entrance cable has the outer sheath degrade to the point where rainwater
gets in and drips down through the meter into the box. In a situation like
that, the service entrance cable should be replaced. Newer cables can get
water in from the weatherhead at the top which drips all the way through to
the panel. The connector at the top of the electric meter is also prone to
taking in water.
The situation is not good. Water can rust out the panel, but it can also
cause arcing inside of the box. It can also cause corrosion on the busbar
and also cause the circuit breakers to fail.
I use GE Silicone 2 Gutter and Flashing caulk to fill in the weatherhead and
around the connector on the top of the electric meter. I also caulk inside
of the meter on the cable that goes down to the panel. Check inside of the
circuit breaker box for moisture on the bus bar. You may have to remove
some circuit breakers. If anything is wet, it will need to be dried. Use
high voltage gloves when drying off the busbar with paper towels. Take a
close look at the main circuit breaker to make sure there is no corrosion on
the terminals.