I was sitting east. After partner's 1♠ I naturally thought of game. When he bid ♦ at the three level I had the feeling he was showing 5 diamonds and 5 spades - clearly I was wrong. Of course with hindsight I should have rebid my hearts: 4♥ would have made and 4♠... well with my partner as declarer, spades breaking 4-2, four down and one of the worst IMPs scores I've had for several weeks.
Not surprisingly this hand led to a monumental 'spat' between me and partner. These things upset me, but every time I get a really bad score I try to look for 'lessons learnt'. I think the main lesson learnt here is, never assume partner has a 5-card suit unless they rebid it.....