Knowing that STP is to prevent switching loops.
Well, after you connect the switches together, the STP goes into its calculation to elect the Root Bridge and block redundant links. Now will the traffic to the next switch never go from blocked to non-blocked port and vice-versa ?
on the screenshot below, if a PC plugged to Switch B wants to reach a PC plugged to Switch D, will the traffic go B to C then to D or straight B to D.
Also the other way around if PC plugged to Switch D wants to reach PC plugged to Switch B, will the traffic go D to C then to B or straight D to B?