Got error 124 from storage engine

[Eric George]

Ok, I've got issues.
I'm getting the above error on insert to my database. It's MySQL 4.1.22 on a RedHat Enterprise 4.0 box. I'm using MyISAM tables.

The table in question has about 19 million records and is part of a merge table with a much larger compressed archive table. This configuration has been working fine for years, with no recent changes.

Despite this error on insertion, the insert appears to complete successfully - my row count goes up, I can query against it (the actual table, and the merge table) and get reasonable results, etc. If I run a check table on this table I get the following:

+-----------------+-------+----------+---------------------------------------------------------------+
| Table | Op | Msg_type | Msg_text |
+-----------------+-------+----------+---------------------------------------------------------------+
| my_db.my_active | check | error | Size of datafile is: 5184194948 Should be: 5184541472 |
| my_db.my_active | check | error | Corrupt |
+-----------------+-------+----------+---------------------------------------------------------------+

If I run a repair table against it, it will tell me it is repaired, and a subsequent check table returns that it's OK. But the same thing happens again on the next insert.

So I tried dumping the table with mysqldump, and then rebuilding it from the dump. Again, the new table looks fine - but as soon as I attempt to insert, I get the same error again??

I found some reference to index size issues when googling this error message. The size of the MYI file on disk is about 824MB - much smaller then some others. There are 4 indexes on the table - one is an auto-increment primary key, two others are single column non-unique indexes, and one is a 3 column unique index involving two ints and a double.