I wouldn't say a slap in the face. There're bound to be differences in opinions and methods, that's why we have many opposition parties instead of one. However, I don't see the various opposition leaders in Singapore slapping each other. SDP may not support certain WP methods, WP may not support certain NSP practices, NSP may not support certain SDP ideologies etc. etc. However, some overzealous supporters from every side may take it personally and exaggerate the differences.
For example, in Malaysia, differences are even more obvious and pronounced. In the PR opposition alliance, PAS wants an Islamic state in their manifesto. DAP wants a secular pluralist state and would never agree to an Islamic state. Irreconciliable, yes? But still both are part of the same alliance that inflicted such damages to BN, their common enemy.
In context of Singapore politics, SDP supporters blaming WP for disunity or WP supporters blaming SDP for disunity. This simply won't work. In the end, who's to blame? Who's to be the judge of who's fault is it?