Some time ago EHP6 was implemented. After some time we realized that the majority of the error messages (collision checks, time quota errors,...) during leave request were not shown anymore. Luckily 2 SAP notes brought us the necessary clarification :
1) OSS Note 1806182 : New class to implement to ensure messages to be shown
2) OSS Note 1070831 : this oss note explains the working of quota checks especially with regard to the chronological order of leave request is multiple requests are done
