Why do I get 80040e30 errors?

When using a command object, or creating tables, you may be using named constants like adVarChar or adDate. If you haven't defined these constants correctly (or forgot to include ADOVBS.inc), you might see this error: 
 

Microsoft JET Database Engine error '80040e30'   The given type name was unrecognized.  or  Type name is invalid.  or  At least one parameter contained a type that was not supported.

 
Make sure your constants are defined correctly (see Article #2112 for better alternatives to including ADOVBS.inc) and, further to that, make sure they're spelled correctly when you use them.