Apparently ERET will throw a COP unusable exception when non-kernel mode.

src/backend/core/interpreter/cop0instructions.cpp

@@ -25,6 +25,10 @@ void Cop0::dmfc0(const Instruction instr) const {
 
 void Cop0::eret() {
   Registers& regs = Core::GetRegs();
+  if (!regs.cop0.kernelMode) {
+    FireException(ExceptionCode::CoprocessorUnusable, 0, regs.oldPC);
+    return;
+  }
   if (status.erl) {
     regs.SetPC64(ErrorEPC);
     status.erl = false;
@@ -76,12 +80,14 @@ void Cop0::tlbw(const int index_) {
 
 void Cop0::tlbp() {
   int match = -1;
-  if (const TLBEntry *entry = TLBTryMatch(entryHi.raw, match); match >= 0) {
+  const TLBEntry *entry = TLBTryMatch(entryHi.raw, match);
+  if (match >= 0) {
     index.raw = match;
-  } else {
-    index.raw = 0;
-    index.p = 1;
+    return;
   }
+
+  index.raw = 0;
+  index.p = 1;
 }
 
 } // namespace n64