We all know that you can't build above 4096m in Second Life, but this building limit has some valid reasoning behind it. Why? Floating point accuracy. Floating Point numbers are, in computer terms, non-whole numbers such as 1.5, 2.625, or 3.14159. Computers are unable to represent them with full accuracy, and a few different systems are used to do so. Each of these systems has numbers that they cannot represent. This results in odd little bugs from time to time, such as prim drift.
These errors are hardly noticeable at ground level, but as you go up in height, they begin to become more apparent. At 5,000m, they become even more noticeable. At 1,000,000 meters up, things are just insane, as you can see below: