I just set up this test harness:

CREATE TABLE dbo.tbl (ID INT, A BIT, B BIT, C BIT);

GO

ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_A DEFAULT(0) FOR A;

ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_B DEFAULT(0) FOR B;

ALTER TABLE dbo.tbl ADD CONSTRAINT DF_tbl_C DEFAULT(0) FOR C;

GO

INSERT INTO dbo.tbl

(ID, A, B, C)

VALUES (1, 1, 1, 1),

(2, 0, 0, 0);

GO

SET XACT_ABORT ON;

BEGIN TRANSACTION;

CREATE TABLE #tmp

(ID INT,

A BIT NOT NULL,

B BIT NOT NULL,

C BIT NOT NULL);

INSERT INTO #tmp

(ID, A, B, C)

SELECT ID,

A,

B,

C

FROM dbo.tbl;

ALTER TABLE dbo.tbl

DROP CONSTRAINT DF_tbl_A,

DF_tbl_B,

DF_tbl_C;

ALTER TABLE dbo.tbl

ALTER COLUMN A DATETIME NULL;

GO

ALTER TABLE dbo.tbl

ALTER COLUMN B DATETIME NULL;

GO

ALTER TABLE dbo.tbl

ALTER COLUMN C DATETIME NULL;

GO

UPDATE t

SET t.A = CASE WHEN x.A = 1 THEN GETDATE()

ELSE NULL

END,

t.B = CASE WHEN x.B = 1 THEN GETDATE()

ELSE NULL

END,

t.C = CASE WHEN x.C = 1 THEN GETDATE()

ELSE NULL

END

FROM dbo.tbl AS t

INNER JOIN #tmp AS x

ON t.ID = x.ID;

DROP TABLE #tmp;

COMMIT;

GO

DROP TABLE dbo.tbl;

Runs without errors. That means there's something else going on with your table. Possibly a trigger, either DDL or DML. That would be my first suspicion in a case like this.

If, for example, there's a logging trigger on column C, but not on A or B, with an audit trail being generated by it, that would totally explain what you're running into.