The following simple script loops forever, even though when stepping through it with the watch window open, the until test (LoopCt==LoopStopNum) is clearly seen to be met (99==99) at the end of the second pass.
Also, it cannot be stopped with Shift-Esc nor Ctrl-B, possibly because the modal message dialog is open most of the time.
Once LoopCt>LoopStopNum, trying to use Quit from the toolbar menu sends the macro into some mode where the modal message dialogs keep popping up but don't wait for input. I had to use task manager to kill the Macro Scheduler program. If the modal message is commented out, it will take over your computer to the extent not even Task Manager can get control; I had to use shutdown from an RDC (Remote Desktop Connection).

One other oddity: when single-stepping, after a MessageModal command, the focus stops reverting back to the "action" window that had the focus just before the message dialog was popped up before each command is executed. This breaks the macro. Workaround - add a SetFocus command after every Message or MessageModal command.
I am using the latest version of Macro Scheduler. I experienced these problems under Windows NT 4.0 TSE and most of them under Window XP Home, as well.
// C:\Program Files\Macro Scheduler\test2b.scp
// this macro demonstrates a bug - loop never ends even though
// LoopCt==LoopStopNum after second pass;
// also, Shift-Esc and Ctrl-B will not stop it;
// you need a Notepad file open named TestDoc.txt
// the easiest way to stop it is to close the TestDoc.txt window
Let>LoopStartNum=97
Let>LoopEndNum=98
Let>VAREXPLICIT=1
Let>WW_TIMEOUT=2
Let>WF_TYPE=2 - Visible Windows Only
CapsOff
Let>LoopCt=%LoopStartNum%
Let>LoopStopNum=%LoopEndNum%+1
Repeat>LoopCt
SetFocus>TestDoc.txt - Notepad
WaitWindowOpen>TestDoc.txt - Notepad
If>%WW_RESULT%=FALSE,WindowError
Send>%LoopCt%
Press Enter
Let>LoopCt=%LoopCt%+1
// warning - if you remove MessageModal on next line or SetFocus at
// beginning of loop, this macro may be very difficult to stop
MessageModal>LoopCt=%LoopCt%
Until>LoopCt,LoopStopNum
MessageModal>Finished
goto>End
Label>WindowError
MessageModal>Window didn't open.
Label>End