acknowledge form was redesigned in ZBX-8820. there is one scenario that is potentially confusing :

• have a trigger with a single event
• start to acknowledge this event, notice how at the options to ack all unacked/problem events it shows "1 event"
• acknowledge this event, then try to acknowledge it again
• notice how the form seems to say that there is 1 unacknowledged event still, even though we just acknowledged the only one.

the count is actually supposed to be the total count of events that will be affected.
maybe the text can be slightly improved to read :
"total of n event[s]"