I have a simple application that adds about 7 million short strings to a HashSet`<string>`. Occasionally I get an exception during a call to Hashset.Add(): System.Collections.Generic.HashSet`1.IncreaseCapacity(): Index was outside the bounds of the array. 

It's an intermittent problem and seems related to memory, but this is on a win2k8 R2 server with 16 GB, not much else going on, most of that physical memory is available. Any ideas?