I haven't been in my house long (since Sept. '08) and it's septic. Just tonight the washing machine drained out into the shower basin of the downstairs bathroom. The downstairs bathroom is actually through the laundry-room. The toilet in the same bathroom did not flush properly right after this occurred. I plunged it only to see more water back up into the shower and hear a gurgling noise coming from the sink next to the toilet. The water in the shower was clear at this point with little or no odor. The water eventually drained and everything seemed back to normal. I assumed it was a temporary clog and that the water pressure had pushed it on through into the tank.
Later, after a shower and using the toilet upstairs, my son came to me and said the downstairs shower was about to overflow. When I rushed downstairs I found a murky brown water standing four inches deep in the shower basin and it was full of sediment but little odor. Also, the water had run under the wall into my garage.
I knocked a hole in the wall in the garage and observed the water leaking from around the seal of the shower basin. This accounts for the water under the wall, but my son tells me that the water has since drained back out. What could cause the water to drain up and down? Why was it clear the first time and murky the second time? Is my septic tank backing up? Is there a busted line? All the lines INSIDE the house are intact and I found no seepage at the joints (except the shower drain). Can anyone give me some advice BEFORE I call a plumber?

I would say your septic tank needs cleaned out. This happened to us before and the tank was full.

What could cause the water to drain up and down? A partial clog or your septic tank's full and it needed time for the drain field to disperse the liquid.

Why was it clear the first time and murky the second time? Who knows? Perhaps the first time only liquid was discharged while the second had solids.

Is my septic tank backing up That would be the first thing I checked. Locate and open up the sewer cleanout, located 18" out from the foundation and brought to grade, flush a toilet and watch the pipe. If you have standing water in it the septic tank should be pumped and then drain field checked out.
If the line's clear then the blockage's in the house and I would snake the washers roof vent. If you do put out enough snake to reach the base and 20 feet more.
But check the septic tank first. Good luck, tom

Thanks, Tom! It was a clog in the stand-pipe. I snaked it and treated the system with some K-47. It's working fine now, great advice.

Glad you got your problem repaired and thanks for the update. Tom