https://www.linkedin.com/pulse/story-behind-negative-float-p6-assem-sherif-pmi-sp-psp

5 – Using multiple calendars

This is obviously a very old bug in P6 that hasn’t been fixed for ages now, if you used multiple different calendars with different hours in the schedule, that may generate a negative float, especially if there are milestones in the schedule, it seems that the late date of the milestone jump to the last working hour of the previous working day in the calendar.

6 – Using Resources dependent activity type

Using the resource dependent activity type with a resource calendar different than the activity calendar may lead also to negative float, this is another bug that primavera needs to work on.

We do not pay attention to negative float values that can be difficult if not impossible to understand in resource leveled schedules, but some clients ask for such calculation so we provide them with the numbers they ask.

Float & Constraints: Jargon - Part 2 : Fieldstone Construction Delay Analyst

Negative Float: A contradiction in terms, which hides the true value of activity float. Float is the extra time available to complete an activity and is best expressed as a positive value. Negative float comes about when the project completion date is imposed on the network. When the project is in delay, the amount of the delay is subtracted from the float. This often, but not necessarily, gives a negative value. Negative float hides the true value of activity float and makes the assessment of relative delay between activities more difficult. If a programme displays negative float it would be better if the true, positive, value was also shown. This would better display where the activity lies relative to other project activities.

If criticality is not well defined in the contract then negative float can determine the entitlement to compensability in concurrent delays.

AACE International Recommended Practice No. 49R-06: Negative Total Float - One of the competing legal applications of concurrent delay says that delay to any work exhibiting negative float that occurs independently and at the same time as another delay may be considered a concurrent delay. The reasoning is that but for the other delay not occurring, then delay to the activity with negative float (and not necessarily the lowest float) would have delayed the project as well. In this case, negative float is used as qualifying criteria for project delay consideration. While this definition of a critical path (and thus concurrent delay) is not as universally used as the concept of lowest total float, it must be considered if the contractual language defines it as the technique to be used.

that is impossible. 

if there are no hard contraints at all I mean NONE 

if the project must finish by date is not set then the backward pass will begin with the latest early finish date 

therefore total float will always be zero