If I start AUTOCAD without setting "limits',What problem will be there ?

You must go to Format icon in the beginning of drawing in order to coordinate the limits of your scene limit of dimensions, limits of leaders , limits of layers and limits of text as well as choosing the units  to prevent any unacceptable problems may occurs during drawing operation    

Limit option in basically,setting up the canvas size where we want the drawing to be drawn. If the value is not set, it will choose the default setting. So it is always good to choose the limits of the drawing while beginning important drawings.

problems will come in drawings dimensions

Drawn object cannot be set to an required scale.

Dimension Problem Actually you got, limits of layers and limits of text as well as choosing the units  to prevent any unacceptable, may occurs during drawing operation

if you start your drawing without limits your drawing will be wrong r without any size or dimensions.

i agree with Muhammad Shakeel  .."As per my previous experience In AutoCAD If any one Start the drawing without setting Limits, there will be no any problem"

I agreed with my all friends But as per my opinion the  Limits Command is basically adjust or fix the drawing area (canvas size)  where a Designer or Drafter can  draw the drawing. Its depends on the required project or assignment. If there is some restricted boundaries so you need to fix the Limits of Drawing area i.e. Topographic Survey, Highway Road Design, Rail, Storm water Layouts  some specific Coordinates and so many But some Architectural and Mechanical, Telecommunication, Electrical drawings, some Detailing and structural details there is no need to set the limits before start the drawings.

As per my previous experience In AutoCAD If any one Start the drawing without setting Limits, there will be no any problem. and i think Most of the user are not using the Limits command only some Drafter or beginner  is using only for exercise purpose.

You need to adjust the some other drawing related requirements ( adjust the units, Dimension styles, Text styles, Layer manager, Limits of Leaders) But these are not the part of Limits Command.

I'm not really AutoCAD person anymore since my last use in college days since discovering3D animations...However, your question reminds me of a joke my Tutor shared.

Young architect had created this amazing drawings then handover the work to construction team, only to realize he had used millimeters instead of meters - result was tiny model.

I'm sure, you wouldn't want to do the same?

LIMITS and UNITS are two differents commands : both have default settings that can be changed, but only the UNITS command is a real deal, because it defines the mesuring units the whole drafting will be in. LIMITS is only used if you want to draw a very big plan, but most of AutoCAD users will never use this command...

Without setting up the limits in AuotCAD is just like giving a piece of cloth to a tailor to make a trouser without giving him any measurement.