If you define a class/object Foo, then later change this to a package, you're left with an error complaining that "Foo is already defined as object Foo," and vice-versa.

To repro, try:

package blah {
object foo
}

Save, then change that to:

package blah {
package foo {}
}

You'll then see the error.

Assigning normal priority since this can be worked around with a Project > Clean.

Using the latest nightly.