Question
Our potted saguaro has gone from the size of a golfball to about 2.5 inches diameter x 8 inches in height in 6 years.  
Now we're afraid it's been overwatered; it's browning and falling over.
The top 1.5 inches are beautifully new-green.
We've tried propping it up, and it can fall in more than one direction.  
Should we bury it up to the green part to stabilize it?  After reading one of the other posted questions, I looked to see whether it's oozing black, and it's not.
Thank you!

Answer
It can take months to years before one can tell that a cactus has died.  It  can look perfectly fine.  Rot often starts in the roots and travels up, and yes, over watering and poorly drained soil don't help.  I suspect your cactus has rot. You may not always see black spots on the plant  If you want to be extra sure, unpot it and look at he roots.  Are they white and healthy looking? Falling over is another give away.  You cactus isn't that big and shouldn't be falling over.  That indicates the internal vascular system is damaged.  The best thing to do, I think, is to cut that green top off above the brown, making sure there is no discoloration in the top part which would indicate rot is still extending up, let it callous over for a week or two and repot that.  The rest, I'm afraid isn't salvigable.  When you cut it you'd probably see some yucky (sorry to be so technical!) black, brown,mushy stuff or just a fragile, dry, almost empty inside.

Cacti only needed to be watered when they are dry, dry.  Then a through watering and wait again till it is very dry.  Also, they are winter dormant so don't require ANY water from Oct. to March and should be kept in a very cool room. Then start gradually watering again.

You didn't do anything wrong.  It just meant that the rot had already extended into the top of the cactus.  It often travels up.  Just for future reference, with your next cactus - and please get another- remember what I said about dormancy.  Try another.  I cna't tell you how many cacti I've killed.