Most of the problems I've seen of this sort are from easy-to-make design issues, like constantly reassigning a string value to modify it (lots of allocations, w

Most of the problems I've seen of this sort are from easy-to-make design issues, like constantly reassigning a string value to modify it (lots of allocations, with lots of GC), or using an IEnumerable with .count() then iterating with foreach (causes multiple enumerations and maybe loads), or unnecessary conversions between List and arrays. If you don't understand what's happening underneath, these mistakes are easy to make. The worst part is these compound over time, and also aren't obvious at small scale. This isn't a problem unique to .net though.