What is the maximum acceptable baud rate error for RS-232 serial communication?
[First off, the "RS" in RS-232 denotes "Recommended Standard", which it was when it was released in the 1960s. Since eons however it is an accepted standard by the EIA as EIA-232, and the old name was officially abandoned since. But it seems there's nothing like killing old habits :-) ]  

On topic: the allowed clock tolerance depends on the detection method. IIRC the old 68HC11 sampled a few times at a bit start, a few times in the middle of a bit, and the at the end. (Sampling occurred at 16x bit rate.) This allows for half a bit difference on the last bit. Most communication on EIA-232 :-) consists of 10 bits (1 start, 8 data, 1 stop), so half a bit in the stop bit is 5%.