In a customers application, a non coded step to set the text of an input field works just fine. Once you convert it to code it fails with:
Exception thrown executing coded step: '[WebTest1_CodedStep1] : @"Enter text '348.000' in 'PnlMainI2GrdItemTxtQuantityText''.
InnerException:
ArtOfTest.WebAii.Exceptions.ExecuteCommandException: ExecuteCommand failed!
InError set by the client. Client Error:
System.ArgumentException: Tag collection is either empty or has less elements than the element occurrence requested. RequestedIndex: '27', ElementLength: '15'
   at ArtOfTest.InternetExplorer.IECommandProcessor.FindElement(ElementIdMessage id)
   at ArtOfTest.InternetExplorer.IECommandProcessor.ProcessActionCommands(BrowserCommand request)
   at ArtOfTest.InternetExplorer.IECommandProcessor.ProcessCommand(WebBrowserClass ieInstance, BrowserCommand request, IHTMLDocument2 document)
BrowserCommand (Type:'Action',Info:'NotSet',Action:'SetText',Target:'ElementId (tagName: 'input',occurrenceIndex: '27')',Data:'348.000',ClientId:'Client_eaa78ec4-67df-4272-8678-03153c9df4ff',HasFrames:'False',FramesInfo:'',TargetFrameIndex:'-1',InError:'True',Response:'System.ArgumentException: Tag collection is either empty or has less elements than the element occurrence requested. RequestedIndex: '27', ElementLength: '15'
   at ArtOfTest.InternetExplorer.IECommandProcessor.FindElement(ElementIdMessage id)
   at ArtOfTest.InternetExplorer.IECommandProcessor.ProcessActionCommands(BrowserCommand request)
   at ArtOfTest.InternetExplorer.IECommandProcessor.ProcessCommand(WebBrowserClass ieInstance, BrowserCommand request, IHTMLDocument2 document)')
InnerException: none.

   at ArtOfTest.WebAii.Core.Browser.ExecuteCommandInternal(BrowserCommand request)
   at ArtOfTest.WebAii.Core.Browser.ExecuteCommand(BrowserCommand request, Boolean performDomRefresh, Boolean waitUntilReady)
   at ArtOfTest.WebAii.Core.Browser.ExecuteCommand(BrowserCommand request)
   at ArtOfTest.WebAii.Core.Actions.SetText(Element targetElement, String text)
   at ArtOfTest.WebAii.Core.Actions.SetText(Control targetControl, String text)
   at MySampleTests.WebTest1.WebTest1_CodedStep1() in C:\Users\gibson\Documents\Visual Studio 2010\Projects\MySampleTests\MySampleTests\Customer Tests\WebTest1.tstest.cs:line 102

Test to repro will be attached to internal bug report.