projectuiapi/src/org/netbeans/modules/project/uiapi/CustomizerPane.form in Design mode displays jLabel1 as having the text

  _C_ategories\:

Similarly, the Resource Bundle editor for its text property shows

  &Categories\:

as the value. However the actual Bundle.properties has

LBL_Customizer_Categories=&Categories\:

where the backslash is a harmless (if useless) escape of the following colon. (Presumably some module in some version of
the IDE inserted the backslash to be conservative, since in a key it needs to be escaped.)

Whatever code reads the property values from the bundle is thus not following java.util.Properties' unescaping semantics.