When I enter a describe table command that also contains comments (using either the /* */ or the double dash "--")
SQL Developer seems to be taking the last word in my comments as the item it is trying to describe.
As a workaround put the comments on a line by themselves - for example BEFORE the desc statement.
Note that placing the DESC on the same line with a comment also does not work in SQL*Plus...
SQL*Plus: Release 22.214.171.124.0 Production on Wed Mar 23 18:03:26 2016
Copyright (c) 1982, 2014, Oracle. All rights reserved.
Oracle Database 11g Express Edition Release 126.96.36.199.0 - 64bit Production
SQL> desc employees /* test */
Usage: DESCRIBE [schema.]object[@db_link]
...so it looks like rp0428's suggestion is best.
Edit: And SQLcl behavior in recent internal builds matches SQL*Plus behavior exactly in this regard.