Oldie but goldie

[valkynaz]

Heard this plenty of times. The way I originally heard it was a hotel not restaurant, and a bellboy not a waiter.

 

[fez]

This relies on the person assuming that £30 is the number to focus on.

£30 in
£27 in after returned 3 quid
Waiter keeps 2. Missing pound.

When £25 is the number to pay attention to.

£30 in
£27 left after returned 3 quid
Waiter keeps 2
£25 left after waiter keeps 2.

No one pockets "extra" pound.