Some people relax for a few days before Ramadhān, intending to busy themselves in 'ibādah once Ramadhān begins. Sometimes this relaxed attitude leads them to sin, with a reassuring feeling that they will make tawbah in Ramadhān.
Firstly, how do they know they will live to see Ramadhān? Secondly, the sin committed may have a negative spiritual impact which may last for the duration of Ramadhān, preventing the perpetrator from repenting and doing good deeds, even in the blessed month.