There's a bug in the Validator Callout in the AJAX Control Toolkit documented by bugs 10900, 12342, 15698, and 11701 on the Code Plex site. All 4 bugs describe the following undesired behavior:
Pull up the AJAX Control Toolkit Sample site's Validator Callout example (here)
Push submit
Note that both fields are invalid, and the first field shows the Validator Callout
Add text to the first TextBox
Push submit
The Validator Callout for the top text field is gone, the Validator Callout...