I actually did a search first to make sure this wasn't already covered. I'll start by saying it was on this forum where a pilot flew to close to his car, had erratic behavior and crashed his bird, I know it seems simple because we are pre-warned about this, and to the OP of that posts' credit, he admittedly knew about this but it still happens. My $.02 is this: First a manhole cover in neighbors yard. In the front of my home is all wavy, my house is like 12' above the road, so all areas aren't level at all, but this manhole covering what I think is a water pipe. It could be the crappy metal they use, and the type and how they mold it can be why, or the pipes themselves could have a small current either on purpose, or even accident running through it. The next thing I came accross is surprising, a big solid concrete park table, again, overlooking the Lake about 300' from my house we have these great heavy unstealable tables (nice area but you can't be too careful and again the only level area to take off, same thing, compass error cannot take off. So, here's the short version for some of you. Some concrete is high enough in magnetic content to produce errors. Manhole covers should be avoided until you know if they are unstable. Now, any of you guys have any places to add that should be avoided? Please contribute if you do, this is a perfect topic to maybe save some poor SOB the heartache and monetary fees associated with fixing their bird, and it could be you!