Can't change a reservation - Conflicting with itself when revisions are enabled

using Calendar View, Inventory, Role Rule Override, Staff, and Template Merci modules

seems to be connected to revisions If I delete the old revisions it will save with no conflict

Seems sporadic as well but I think I have it narrowed down to the third change with a specific item selected so:
confirm, check out, check in (no errors)
However:
confirm, confirm with items selected, checkout, Check in (Error)

confirm, confirm additional item added, confirm with items selected, checkout, Check in (Error)

confirm,confirm (with items selected), confirm additional item added , confirm with second item selected (Error)

Temporary band aid
As long as you are using the ajax validation via Merci Staff, you should never see the conflict grid anyways.
Just comment out lines 100 to 104 in api.inc

      if (!$count or (empty($item_nid) and merci_type_setting($type) == 'bucket' and $count - $selected_count[$type] < 0)) {
        $messages[$did] = merci_theme_conflict_grid($type, $title, $start, $end, $item_nid, $node->vid);
        form_set_error("merci_reservation_items][$did][merci_item_nid", $messages[$did]);
        continue;
      }

The patch seems to be functioning properly.
Thanks for looking at this so quickly with

Almost there.
Here is the new scenario:
Item is checked out for a full day it comes back early and is checked in.
New reservation is created that item is available and selected. Conflict grid error appears.
Correct the to time on the checked in reservation. Still errors
Delete revisions of the checked in reservation. Works fine

Initial testing looks good

We charge for late fees so changing the date due when it comes back isn't something we do. I don't understand why this is ideal, seems like it would skew usage data? I created a separate Checked in date field that is auto completed via a triggered rule on changing the status to Checked in. Anyway seems to be mute see #1512294: Double Check out Error Haven't seen any other issues regarding the conflict grid