Create random-notes.md

Smart-Contract-Security/random-notes.md

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+## Random security notes
+
+<br>
+
+* `tx.origin` is used: you want to replace it by “msg.sender” because otherwise any contract you call can act on your behalf.
+* Avoid potential reetrancy bugs: 
+```
+msg.sender.transfer(amount);
+balances[msg.sender] -= amount;
+```
+* Inline assembly should be used only in rare cases.
+* Unclear semantics: `now` is alias for `block.timestamp` not current time; use of low level `call`, `callcode`, `delegatecall` should be avoided whenever possible; use `transfer` whenever failure of ether transfer should rollnack the whole transaction.
+* Beware of caller contracts: `selfdestruct` can block calling contracts unexpectedly.
+* Invocation of local functions via `this`: never use `this` to call functions in the same contract, it only consumes more gas than normal call.
+* Transferring Ether in a for/while/do-while loop should be avoid due to the block gas limit.
+* ERC20 `decimals` should have `uint8` as return type.
+
+<br>