When working with Outlook Calendar's API or building integrations, you might encounter recurring events displaying a strikethrough through the recurrence symbol (↻). This isn't a UI bug - it's actually an important status indicator that affects how you should handle the event programmatically.

The line through the recurrence symbol indicates that this particular instance of a recurring series has been modified or canceled while remaining part of the series. From a developer's perspective, this translates to specific properties being set in the underlying event object:

// Example of a modified recurring event in Outlook REST API response
{
  "id": "AAMkAG...",
  "subject": "Modified Team Standup",
  "start": {"dateTime": "2023-06-15T09:00:00", "timeZone": "Pacific Standard Time"},
  "isCancelled": false,
  "type": "exception",  // Key property
  "seriesMasterId": "AAMkAG...",
  "originalStartTime": "2023-06-15T09:00:00"
}

When processing calendar events programmatically, you need to handle these exceptions differently:

1. API Behavior: The Outlook Graph API returns these as separate events with type="exception"
2. Data Integrity: Modifying one instance doesn't affect the entire series
3. Sync Challenges: These require special handling in synchronization scenarios

Here's how to properly handle recurring event exceptions when working with the Outlook API:

// C# example using Microsoft Graph SDK
var events = await graphClient.Me.Calendar.Events
    .Request()
    .Header("Prefer", "outlook.timezone=\"Pacific Standard Time\"")
    .GetAsync();

foreach (var calendarEvent in events)
{
    if (calendarEvent.Type == EventType.Occurrence ||
        calendarEvent.Type == EventType.Exception)
    {
        // Handle modified/canceled recurring instances
        var masterId = calendarEvent.SeriesMasterId;
        var originalStart = calendarEvent.OriginalStartTime;
        
        Console.WriteLine($"Modified instance of series {masterId} at {originalStart}");
    }
}

• Assuming all events in a series have identical properties
• Not checking the OriginalStartTime when processing exceptions
• Forgetting to handle deleted instances (where isCancelled=true)

For applications that need to track changes to calendar events, you can use delta queries with special handling for recurring series:

// JavaScript example using delta queries
const response = await client
    .api('/me/calendarview/delta')
    .query({
        startDateTime: '2023-06-01T00:00:00',
        endDateTime: '2023-06-30T23:59:59'
    })
    .get();

response.value.forEach(event => {
    if (event.type === 'exception') {
        // Process modified recurring instance
        trackSeriesModification(event.seriesMasterId, event.originalStartTime);
    }
});

When working with Outlook Calendar's API or automating calendar events, you might encounter the recurring event symbol (↻) with a strikethrough. This occurs in three specific scenarios:

// Example in Outlook REST API response
{
  "eventId": "AAMkADk0...",
  "subject": "Weekly Sync",
  "isRecurring": true,
  "exceptionDetails": {
    "isCancelled": true,
    "originalStart": "2023-07-15T10:00:00"
  }
}

The strikethrough indicates a modification to the recurrence pattern at the API level:

1. Single Instance Cancellation: When using EWS (Exchange Web Services), the ResponseType property shows "Declined" for that instance
2. Modified Occurrence: The ModifiedPropertyList contains changes to that specific recurrence
3. Series Termination: The Recurrence property's EndDate is set to before this occurrence

When processing calendar events programmatically, check these properties:

// PowerShell example using EWS
$service = New-WebServiceProxy -Uri "https://outlook.office365.com/EWS/Exchange.asmx"
$events = $service.FindItem(
    [Microsoft.Exchange.WebServices.Data.WellKnownFolderName]::Calendar,
    [Microsoft.Exchange.WebServices.Data.ItemView]::new(100)
)

foreach ($event in $items) {
    if ($event.Appointment.IsRecurring -and $event.Appointment.IsCancelled) {
        Write-Host "Found cancelled recurrence instance:" $event.Subject
    }
}

When building calendar integration:

• Always check the 'type' field in Graph API responses - "exception" indicates modified occurrences
• In EWS, examine the AppointmentState bitmask for the sfException flag (value 1)
• For sync operations, track the originalStartDateTime field to properly map modifications

Here's how to handle modified recurrences using Microsoft Graph:

// JavaScript example
const options = {
  headers: {
    Authorization: Bearer ${accessToken}
  }
};

const response = await fetch(
  'https://graph.microsoft.com/v1.0/me/events?$filter=isRecurring eq true',
  options
);

const events = await response.json();
events.value.forEach(event => {
  if (event.type === 'exception') {
    console.log('Modified recurrence:', event.subject);
    console.log('Original date:', event.originalStart);
  }
});

Remember that the strikethrough is just UI representation - the underlying data structure contains all necessary flags to programmatically identify these special cases.