Looking at the RAF there are several campaigns that could be described as (at best) unsuccessful: night defences until Spring 1941, the strategic bombing offensive until 1943, Fighter Command 'leaning forward into France' after the Battle of Britain, the fall of Greece/Singapore/Burma. Unsustainable losses were suffered in unescorted bombing early in the war (e.g. the Battle of the Heligoland Bight, where 12 of 24 Wellingtons were lost); RAF and FAA aircraft suffered heavy losses and failed to sink Scharnhorst and Gneisenau in the Channel Dash; the air clashes over the Dieppe Raid were inconclusive, the Allies suffering greater losses but generally preserving air superiority over the landings.
To focus on a specific example, though, the heaviest loss rate in a single operation was on May 14th 1940. The spearhead forces of the German invasion of France crossed the Meuse River around Sedan on May 13th, and though several bridges were destroyed by the defenders German engineers constructed pontoon bridges to allow mechanised units to cross and exploit the bridgehead.
On May 14th the aircraft of the RAF's Advanced Air Striking Force (AASF) were committed in an attempt to destroy the German bridges and halt the crossing. 10 Fairey Battle light bombers attacked early in the morning from high altitude, suffering no losses but causing no damage. French bombers attacked around noon, suffering losses and again not destroying the bridges. Between 1500 and 1600 hours the entire available AASF bomber force attacked, 71 aircraft in total. Luftwaffe fighters were now present and the aircraft attacked from low altitude, rendering them vulnerable to flak defences. The result of the attack was:
| Wing | Squadron | Aircraft Type | Despatched | Lost |
|---|---|---|---|---|
| No. 76 | No. 12 | Fairey Battle | 5 | 4 |
| No. 76 | No. 142 | Fairey Battle | 8 | 4 |
| No. 76 | No. 226 | Fairey Battle | 6 | 3 |
| No. 71 | No. 105 | Fairey Battle | 11 | 6 |
| No. 71 | No. 150 | Fairey Battle | 4 | 4 |
| No. 71 | No. 114 | Bristol Blenheim | 2 | 1 |
| No. 71 | No. 139 | Bristol Blenheim | 6 | 4 |
| No. 75 | No. 88 | Fairey Battle | 10 | 1 |
| No. 75 | No. 103 | Fairey Battle | 8 | 3 |
| No. 75 | No. 218 | Fairey Battle | 11 | 10 |
In the evening 28 Blenheims of No. 2 Group Bomber Command made a final attack; six were lost.
The attacks were unsuccessful, failing to destroy the bridges or seriously disrupt the German advance. Overall losses were comparable or greater in some of the examples in the first paragraph, but with many more aircraft participating; according to the official historian "No higher rate of loss in an operation of comparable size has even been experienced by the Royal Air Force."
See:
The Royal Air Force 1939-1945 Volume I: The Fight at Odds, Dennis Richards
The War in France and Flanders 1939-1940, Major L. F. Ellis