Mark
I know from my experience that depending on what the object is, how corroded it is and the what the soil is comprised of makes it harder to get an accurate hit sometimes. The microprocessor in today's MT's are great, but if an object has been in the ground for a long time, corrosion will fool the MT into thinking the object is something else. I have had it happen to me plenty of times.
When I first got my MT, I went into my backyard with different coins, bolts, nuts. all kinds of stuff, and dug holes all over the place and put in the objects, (my wife wasn't to happy) and covered them up to get a feel of my MT. It helped me allot however. they were all nice and clean. I thought at the time that metal detecting would be a breeze. I found out real quick that is not the case. Metal detecting is a continuous learning process.
Take old coins and stuff you find, don't clean them, leave them dirty and corroded and bury them in the ground under a controlled environment. Try putting in the coins flat, on edge, and an at different angles, even put totally different metal objects together in the same hole.
Oh. When you do this, (and even when you hunt) I have found that plastic golf tee's are great to mark spots. Having the tee right over the object also helps you to learn good pin-pointing habits, and doesn't affect the MT.
Here is a pretty nice site on metal detecting if you are interested.
metal detecting world dot com. (When I pasted the link, it didn't show up right. the link is one word. I had to space it to show up)
Check out the "Metal Detecting Secrets For Beginners" even if you are not a beginner. The part in there about "Halo Effect & Disappearance of A Good Signal" sure cleared up allot of questions I had when I got hits, dug and dug and didn't find anything.
I have had my MT for a couple of years and I am definitely not an expert. I still search the WEB on metal detecting for tips tricks and such. Plus. This forum has a great many of knowledgeable people are willing to help.
Happy hunting.
Oh. And lets us know if you try my suggestions and if if helps you any.