Is language equality for linear context-free grammars decidable?
Quoting from Amiram Yehudai, The Decidability of Equivalence for a
Family of Linear Grammars, Information and Control 47, 122-136 (1980), page 1:


  The equivalence problem for various families of languages is of great
  interest in the theory of formal languages. This problem is decidable
  for regular languages (Rabin and Scott, 1959) and undecidable for
  context-free languages (Bar-Hillel et al., 1961). It is also
  undecidable for the family of linear context-free languages, as
  follows from Lemma 1 in (Baker and Book, 1974). The family of uniform
  linear languages is a natural and nontrivial subfamily of the linear
  languages for which equivalence is decidable.


This refers to Baker, B. S. and Book, R. V. (1974), Reversal-bounded multipushdown machines, J. Comput. System Sci. 8, 315-332, which, in the proof of that Lemma 1, presents a subset of linear context-free languages such that deciding whether a member of the set is equal to $\Sigma^*$ is equivalent to deciding the Post Correspondence Problem.