Per discussion below, a message should be added to Special:Watchlist to alert users that /raw and /clear use the Job queue and it may take a while.
--Original Report--
When clearing my watchlist it took a while to update and show no pages were on there
Method 1:
1. Visit Special: Watchlist
2. Click the edit button
3. Then clear this watchlist
4. Press the big red confirm button
5. See an error stating too many pages to display
{F28931371}
6. Return to your watchlist - It will not be cleared
7. Wait about an hour and return to your watchlist. It should be clear
Method 2:
1/2: As above
3. Click edit raw watchlist
4. Select everything in the box and blank it
5. Save changes
6. As above (5/6/7)
NB: This is displayed on enwp with 1428 pages (excluding talk) on the Watchlist being cleared