Why am I getting an ORA-01722 (invalid number)?

Question

I've been using a parameterized query to insert values into an Oracle table, like so:

var q = "insert into MyTable(Field1, Field2...) values(:Field1, :Field2...)";
var cmd = new OracleCommand(q, conn); // conn is a pre-existing connection
cmd.Parameters.Add("Field1", field1Val); 
cmd.Parameters.Add("Field2", field2Val);
// etc...
cmd.ExecuteNonQuery();

This has been working fine, but suddenly this has stopped working, and I am getting Oracle error ORA-01722 (invalid number). I have checked the parameters, and all numbers are unquestionably valid numbers. I even substituted dummy values for any nulls, and I still get the error. I tried the same query in direct sql (using OraDeveloper Studio), and it works, even with the identical parameters.

How do I track this one down?

EDIT: per request in the comments, here's the create table statement:

CREATE TABLE ALPHA.VISITFINDINGS (
  ID NUMBER(12),
  VISITID NUMBER(12) NOT NULL,
  DESCRIPTION VARCHAR2(100),
  CUSTOMIMAGE CLOB,
  VISUALFINDINGSSECTIONMAPID NUMBER(12),
  FINDINGSID NUMBER(12),
  CONSTRAINT FK_VISITFINDINGS_AREA FOREIGN KEY (VISUALFINDINGSSECTIONMAPID)
    REFERENCES ALPHA.VISUALFINDINGSSECTIONMAP(VISUALFINDINGSSECTIONMAPID),
  CONSTRAINT FK_VISITFINDINGS_FINDINGS FOREIGN KEY (FINDINGSID)
    REFERENCES ALPHA.FINDINGS(FINDINGSID),
  CONSTRAINT FK_VISITFINDINGS_VISIT FOREIGN KEY (VISITID)
    REFERENCES ALPHA.VISITS(VISITID),
  CONSTRAINT PK_VISITFINDINGS PRIMARY KEY (ID))
TABLESPACE USERS
STORAGE (
  INITIAL 64K
  MAXEXTENTS UNLIMITED
)
LOGGING;

Answer 1

I have given answer credit already, but I think it's worth mentioning here exactly what the root of my problems was, in case anyone else finds this item while looking for an answer to their own problem.

The problem is that the C# implementation of parameterized queries for Oracle contains a serious and potentially dangerous bug - a real "pit in the public domain":

It doesn't matter what you name your parameters; they have to be added in the order in which they appear in the query.

See more here.