another common but dangerous thing is people think they can stop up onto a log that has landed on the beach. they think they can let the wave come up around them, they stay dry and it's really neat. the water can lift or just jar a big log , the log rolls a bit and the person winds up stuck beneath it.  most often the person can slip out from under it, but not always. i remember once it happened to a U of O guy and the tide was rising, basically overtook him. he did drown. 
 sand caves ,or "forts", on the beach or the 'wall" - bad.. it's good. it's good and then it falls in.