Hi, I have a test case like that: -=-=- testIncrementBy prefix := MyObject new numbers: #(1 10); yourself. prefix incrementBy: 10. self assert: prefix numbers equals: #(1 20). -=-=- and the byte code starts like that: -=-=- 57 <40> pushLit: MyObject 58 <CC> send: new 59 <88> dup 60 <21> pushConstant: #(1 10) -=-=- If I run the test case, it is green, but the byte code changes into: -=-=- 57 <40> pushLit: MyObject 58 <CC> send: new 59 <88> dup 60 <21> pushConstant: #(1 20) -=-=- The #incrementBy: method looks like: -=-=- incrementBy: aNumber numbers at: 2 put: (numbers at: 2) + aNumber -=-=- Is it correct that it also changes the method byte code? I can put "#(1 10) copyâ into the test case. I am just curious if it is a feature or bug. Tested in Pharo4.0, Latest update: #40613. Cheers, Juraj