Explanation:

This is an unexpected error, possibly caused by an internal error in Foxhound.

Please report this error; see How do I report unexpected errors and exceptions?

If possible, include the diagnostic message and associated dump; here is an example:

1. Use the Foxhound Options - 8. Diagnostics - Display Diagnostics button to find the exception id, 477 in this case:

 477 - 2012-07-26 06:56:37.407 Full Build 4173a 1000001557 203a3d3(203eh1) An attempt to start sample loop event 1000001557 for sampling id 4 failed because event 1000000020 was already running for that sampling id. [4-Foxhound_on_old_Dell_XP]

2. Run adhoc queries to see the diagnostic message and the associated dump (see How do I run adhoc queries on the Foxhound database?):

SELECT * FROM exception_diagnostic WHERE exception_id = 477;
SELECT * FROM exception_dump       WHERE exception_id = 477;

exception_id,exception_diagnostic
477,2012-07-26 06:56:37.407 Full Build 4173a 1000001557 203a3d3(203eh1) An attempt to start sample loop event 1000001557 for sampling id 4 failed because event 1000000020 was already running for that sampling id. [4-Foxhound_on_old_Dell_XP]
SELECT * FROM exception_dump WHERE exception_id = 477;

exception_id,dump_id,dump_inserted_at,dump
477,1,'2012-07-26 06:56:37.409','rroad_sampling_options','203a3d3','(203eh1)',4,'1','Foxhound_on_old_Dell_XP','Foxhound_on_old_Dell_XP','',,,,'Y','N',2012-07-23 12:09:23.973,'OK',1000000020,'','Y','',0,4,2012-07-26 06:56:36.070,2012-07-26 06:56:36.070,2012-07-26 06:56:26.026,2012-07-26 06:56:26.030,'12.0.1.3298','12.0.1.3298','p001',8192,'N','N','N','DBA','',2147483647,'Conn #','ASC',0,'',''\x0d\x0a

3. Run adhoc queries to copy the diagnostic message and associate dump to a single text file:

UNLOAD SELECT * FROM exception_diagnostic WHERE exception_id = 477 TO 'c:/temp/foxhound_exception.txt';
UNLOAD SELECT * FROM exception_dump       WHERE exception_id = 477 TO 'c:/temp/foxhound_exception.txt' APPEND ON;

477,'2012-07-26 06:56:37.407 Full Build 4173a 1000001557 203a3d3(203eh1) An attempt to start sample loop event 1000001557 for sampling id 4 failed because event 1000000020 was already running for that sampling id. [4-Foxhound_on_old_Dell_XP]'
477,1,2012-07-26 06:56:37.409,'''rroad_sampling_options'',''203a3d3'',''(203eh1)'',4,''1'',''Foxhound_on_old_Dell_XP'',''Foxhound_on_old_Dell_XP'','''',,,,''Y'',''N'',2012-07-23 12:09:23.973,''OK'',1000000020,'''',''Y'','''',0,4,2012-07-26 06:56:36.070,2012-07-26 06:56:36.070,2012-07-26 06:56:26.026,2012-07-26 06:56:26.030,''12.0.1.3298'',''12.0.1.3298'',''p001'',8192,''N'',''N'',''N'',''DBA'','''',2147483647,''Conn #'',''ASC'',0,'''',''''\x0d\x0a'

See also...
How do I report unexpected errors and exceptions?
How do I run adhoc queries on the Foxhound database?
How do I see the schema for adhoc reporting?