Yes, mixing elapsed duration will interrupt critical path - as when you are using elapsed time, the task could end during non-working time. For example (and really more for people reading this after - it sounds like you have it sorted) I have 3 tasks all linked F to S. Task 1 has a duration of 1 day, Task 2 is 12 h, and task 3 is one day. All tasks are critical. I change the duration of task 2 from 12 h to 12 elapsed h. The task now begins at 5:00 pm (following the end of Task 1) and ends 12 hours later at 5:00 am. Task 3 doesn't start until 8:00 am because that's is the start of the working day. Task 2 is not critical because of the 3 hours of slack between the finish of Task 2 and the start of Task 3.