blarg.csx

using System.Globalization;
using System.Threading;

var test = "aaaaaaaaaaaaaaaaaaaa";

var cultures = CultureInfo.GetCultures(CultureTypes.AllCultures);
						  
foreach(var culture in cultures)
{
	Thread.CurrentThread.CurrentCulture = culture;
	for(var aCount = 1; aCount <= test.Length; aCount++)
	{
		var checkString = new String('a', aCount);
		if(!test.StartsWith(checkString))
		{
			Console.WriteLine("{0} fails for string {1}", culture.Name, checkString);
		}
	}
}

output

da fails for string a
da fails for string aaa
da fails for string aaaaa
da fails for string aaaaaaa
da fails for string aaaaaaaaa
da fails for string aaaaaaaaaaa
da fails for string aaaaaaaaaaaaa
da fails for string aaaaaaaaaaaaaaa
da fails for string aaaaaaaaaaaaaaaaa
da fails for string aaaaaaaaaaaaaaaaaaa
da-DK fails for string a
da-DK fails for string aaa
da-DK fails for string aaaaa
da-DK fails for string aaaaaaa
da-DK fails for string aaaaaaaaa
da-DK fails for string aaaaaaaaaaa
da-DK fails for string aaaaaaaaaaaaa
da-DK fails for string aaaaaaaaaaaaaaa
da-DK fails for string aaaaaaaaaaaaaaaaa
da-DK fails for string aaaaaaaaaaaaaaaaaaa
kl fails for string a
kl fails for string aaa
kl fails for string aaaaa
kl fails for string aaaaaaa
kl fails for string aaaaaaaaa
kl fails for string aaaaaaaaaaa
kl fails for string aaaaaaaaaaaaa
kl fails for string aaaaaaaaaaaaaaa
kl fails for string aaaaaaaaaaaaaaaaa
kl fails for string aaaaaaaaaaaaaaaaaaa
kl-GL fails for string a
kl-GL fails for string aaa
kl-GL fails for string aaaaa
kl-GL fails for string aaaaaaa
kl-GL fails for string aaaaaaaaa
kl-GL fails for string aaaaaaaaaaa
kl-GL fails for string aaaaaaaaaaaaa
kl-GL fails for string aaaaaaaaaaaaaaa
kl-GL fails for string aaaaaaaaaaaaaaaaa
kl-GL fails for string aaaaaaaaaaaaaaaaaaa

Which cultures are those? "da" or "da-DK"?

See here for a more complete repro: http://stackoverflow.com/questions/15547663/aaaa-startswithaaa-returns-false

da and kl since I'm explicitly excluding specific cultures

var cultures = CultureInfo.GetCultures(CultureTypes.AllCultures)
.Except(CultureInfo.GetCultures(CultureTypes.SpecificCultures));

Revision 3 brings in multiple length checks

Note it only fails on odd-length strings

Revision 4 shows all cultures since @ploeh asked about culture vs culture-specific. No change in output, but have included it for completeness.