See _refundToken() method:
The transfer(...) function forwards a fixed amount of 2300 gas. Historically, it has often been recommended to use these functions for value transfers to guard against reentrancy attacks. However, the gas cost of EVM instructions may change significantly during hard forks which may break already deployed contract systems that make fixed assumptions about gas costs. For example, EIP 1884 broke several existing smart contracts due to a cost increase of the SLOAD instruction. See here.
Use call(...) to prevent potential gas issues.
