1. Including ink in the box. Apparently, nobody does this except for most of the boxers in my locale. Ink is really expensive, it gets moldy (above photo) or dried out, it bulks up the box contents so you need a bigger box, and if the box leaks the entire contents can become covered in colored water. Letterboxers often don't even use the ink -- they use their own. So why bother?
2. Using old fashioned Rubbermaid-style containers, even high quality expensive ones ("Housatonic Forest"). They are not designed to have any weight on them and the seal breaks if a rock is placed on them or they are distorted in any way. I only use the heavy rigid Lock & Lock style boxes now, with four locking tabs. A popular source for a more cost effective version of Lock & Lock for letterboxing is Ocean State Job Lot, who sells a nice 13.5 oz "Freshness Keeper" for just $1.25 each.
3. Using duct tape to conceal a box ("Shelton Canal"). It takes a long time to do, and tape comes off in a sticky way. It's also pretty visible in the winter. Now I use a very thin layer of flat black spray paint formulated for plastic. You're hiding a box in shadows, so a dark box disappears best. I've also seen people use dark brown. One nice thing about spray paint is you can do a lot of boxes up in just a couple of minutes. One option is to leave the purchase sticker on while painting and remove it to form a window. That's something to keep in mind if you're planting in an area where the Secret Service might become suspicious of your box and think it's a bomb. Better yet, don't plant there.
4. Planting too close to the trail (Riverview Park). I now assume that every box will become exposed. Someone won't rehide well, an animal might dig it up, or a heavy rain might wash away the cover. A box next to the trail will be seen eventually, and likely tampered with. I now try to hide far enough off trail so that if the box is uncovered, the box still won't be seen.
5. Planting in a drainage area ("Housatonic Forest"). Water washed those boxes right out of their hiding hole! Water can also wash a box down into a crevice where it can't be reached.
6. Planting in a really neat location that is nearly impossible to write clues for ("Lucy the Fox", for sure). I'm learning to plant near something really distinctive. Doesn't always work, but that's the goal.
7. Using cheap plastic bags, especially sandwich bags. They rip, they leak. Best ones are heavy duty freezer bags with double seals. Pint size is best, but very hard to find. I usually carry the freezer bags with me while boxing and replace old flimsy bags that I find.
8. Planting in an animal den. Found the box 50 feet away unopened ("Pine Lake").
9. Including a pen or pencil in the box. You know what? Boxers are supposed to carry that stuff with them when they go looking for boxes. All a letterbox needs is a stamp, a logbook, and a plastic bag to keep the two dry.
10. Planting in a patch of poison ivy during the winter. Oops. ("A Very Long Beach").
I've made plenty more mistakes! But I'll just stick with ten here.
8. Planting in an animal den. Found the box 50 feet away unopened ("Pine Lake").
9. Including a pen or pencil in the box. You know what? Boxers are supposed to carry that stuff with them when they go looking for boxes. All a letterbox needs is a stamp, a logbook, and a plastic bag to keep the two dry.
10. Planting in a patch of poison ivy during the winter. Oops. ("A Very Long Beach").
I've made plenty more mistakes! But I'll just stick with ten here.