Testing a grammar converted from other parser generator tool I ended up commenting something inside a rule and discovered that lemon can't handle this particular comment:
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prog ::= stmt /*=*/ .
prog ::= stmt2 /*/=*/.
stmt ::= .
stmt2 ::= .
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Lemon output:
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../lemon gram.y 
gram.y:2: Illegal character on RHS of rule: "=".
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But with this grammar (notice add a space here "/* /=*/"):
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prog ::= stmt /*=*/ .
prog ::= stmt2 /* /=*/.
stmt ::= .
stmt2 ::= .
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Lemon output:
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../lemon gram.y 
gram.y:4: This rule can not be reduced.

1 parsing conflicts.
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