Bechamel:
If your Bechamel is turning lumpy while cooking, hold the milk and add ice-cold water. Continue to whisk briskly until lumps disappear.
Hollandaise:
Even for experts, hollandaise separates. Try whisking in a little boiling water, drop at a time; If not successful, put another yolk in a separate bowl and whisk in separated sauce slowly!
Mayonnaise:
If your mayonnaise starts to split try whisking in a little boiling water, drop at a time; If not successful, start whisking another egg yolk in a separate bowl with some vinegar and slowly add the split batch to it.
Custard (Sauce Anglaise):
If your custard develops lumps or is starting to curdle add an ice cube and give it a quick blitz in the blender.